That depends on what client you are using here on Nostr the social network apps that let you access nostr we call are clients. Every client can offer you different features & give you a different experience at the way you look at Nostr. I don't know if you have iphone or android I use Amethyst to connect to Nostr this client Amethyst is an android app & to change an avatar on that you just go to your profile hit the edit profile button & you can upload your avatar from there.

If you are on iphone you might be using Damus which I am not familiar with but I believe it is the same way you go to your profile you should see an icon there like a pencil next to paper & from there it may be to edit your avatar.