People usually greet each other with a warm good afternoon at a particular time of the day. This greeting is appropriate immediately after noon and before evening.
A good afternoon is frequently used as a greeting in many countries where English is spoken. So how do you reply to “Good afternoon”.
The best response to Good Afternoon is to return the greeting with the person’s name who greeted you, for example: “Good afternoon, Jon.” Return the courtesy even if, strictly speaking, it is not an afternoon at that moment.
Use the Same Phrase
If someone greets you with good afternoon, it’s often a good idea to reply by using the same phrase. Simply saying any of the following can help to brighten the other person’s day:
“Good afternoon. How are you?”
“Good afternoon to you as well.”
If the person who initially greeted you seems to be in a hurry, you can stop at good afternoon. You won’t need to inquire after their health or exchange other pleasantries if they were simply being polite but don’t seem to have the time for a longer conversation.
Similarly, an extended sentence may not be appropriate with all people or in all situations. For example, if you are greeting many people in a crowded room individually, you may not have the time to say more than a brief good afternoon to each other as you walk by.
Add Their Name
Human beings like to hear their name. To many people, that’s one of the most beautiful words in any language.
Dale Carnegie, an American writer & lecturer (1888-1955), always encouraged business teams and salespeople to use the name of a client whenever they were speaking to that person. That’s done because to the client, their name is the sweetest sound that they could ever hear.
Your brain voluntarily responds to your name.  Adding a person’s name to your greeting can help them to associate you with a positive feeling.
You could say:
“Good Afternoon, Mila.”
“Good Afternoon, Roger. How are you today?”
“Good Afternoon, Ichiro. It’s good to hear from you.”
If you don’t know their name and you wish to greet them, you can do so. For example, you could say:
“Good afternoon, ma’am.”
“Good afternoon, sir.”
“Good afternoon, miss.”
Always use a polite tone when you’re replying to someone else’s greeting. Don’t be dismissive, caustic, or sarcastic if you want to make a good impression.
Sometimes you might unconsciously introduce other emotions into your greeting because of what you’re experiencing. For example, if you’re a busy sales representative who is dealing with a customer and someone else passes and greets you, you might respond absentmindedly.
However, your greeting at that point might not be injected with the level of courtesy that would usually be standard for you. If possible, it’s good for you and others to take a quick breather now and then.
This helps to keep your mind calm and it’s good for your overall well-being. In addition, it changes the way that you relate to others when you give yourself a moment to breathe and you’re not constantly under stress.
Add a Smile
Whether you’re on the phone or speaking to a person face to face, it’s important to add a smile to your voice when you’re returning their greeting.
You may be tempted to think that if they’re on the phone they can’t tell whether you’re smiling. However, that’s far from the truth.
Whether it’s a personal or business call, you should add a smile to your voice when you’re speaking to a person on the phone who greets you with a good afternoon. They won’t be able to see your face but they’ll be able to hear the warmth in your tone.
Smiling affects the way we speak.  When your good afternoon is complemented by a smile, the people who are listening to you will reflexively smile at you. This response is unconscious but researchers have shown that this is what happens. 
Ensure that your smile is genuine. People can detect the differences among several types of smiles. Human beings can distinguish among up to 50 different types of smiles. So, if you want to make an impact with your pleasant afternoon greeting, smile with positive thoughts towards the other person.
Reply with Another Greeting
You might not always feel like saying good afternoon in response. In most situations, it’s okay to use another form of greeting. This is especially true in more casual situations.
For example, you could say good day instead of a good afternoon. You could also say hello or say hi.
However, your best bet is always to respond with the same form of greeting that was initially used.
Use Nonverbal Replies
There might be occasions when you can’t speak. For example, if your mouth is full and someone says good afternoon to you, it wouldn’t be appropriate to answer verbally at that point.
You could simply smile at the person in response while ensuring that you make eye contact. You could also nod or wave at them.
All of these nonverbal responses are appropriate when you’re unable or unwilling to respond verbally for some reason.
What If It’s Not Afternoon?
Many people technically regard the afternoon as the time between 12:00 noon and 6:00 p.m. They use the good morning to greet others before noon.
Similarly, some people tend to use a good evening after 6:00 p.m. However, the understanding of when afternoon ends and good evening would be a more appropriate greeting varies from one person to another.
If someone greets you with a good afternoon when you think a good evening would have been better, you can sit still simply reply with a good afternoon. If you do decide to reply with a good evening, ensure that your tone is friendly, so you don’t come across as unnecessarily correcting a person who is simply trying to be courteous.
People don’t often check their watches before they greet another human being. If someone has misjudged the time, that’s not really what’s important. They’re simply using a greeting to acknowledge that you’re in the space and it’s important for you to show similar respect.