This is my 2nd time playing Final Fantasy X and its been a while so I forgot how to edit and upgrade Aeons abilities. I know it used to be so obvious but I just cant find it and i checked multiple times and asked one of my friends but he forgot to. Oh and im pretty sure its in the menu screen somewhere. If you can help thanks a lot.

Just go to the Aeon section of the menu. You'll need the Summoner's Soul before you can upgrade your Aeon's abilities and stats however.

You have to get Summoner's Soul from Belgemine (sp?) after you visit the Djose Temple. She is on the right side of the road that leads to the Moonflow.

Oh I didnt get there yet that explains it Thanks