I bought the head texture, hit the customize button and uploaded my texture and it said it uploaded. However, the texture isn't showing in the game. I don't know what to do. (P.S. This is my first time uploading a texture so I don't know if I'm doing this entirely correct.)

have you tried to type /dl Abigigail in game?

Do you have body textures activated ? To do so just got to settings/ effects.