Easily? Find a tool someone else made for it.
If that has not happened (and nobody tends to really care about annual sports games, much less for the handhelds) then yeah it is going to involve learning a bit.
For the GBA and looking at some gameplay I would expect most kits to instead be palettes rather than sprites.https://web.archive.org/web/20081224044418/http://etk.scener.org:80/?op=showtutorial&st=3
You can check fairly quickly with the OAM and palette viewers that the tutorial above will make use of.
At the same time there are that many teams (each with home and away or alt kits and a goalie?) that I could imagine it being a more dynamic affair -- while many games will be like the above and load a single palette and have the character wear the same thing for the whole game then here with the amount of combinations it is not impossible that the devs generate what they need on the fly.
I cover a bit more specific GBA graphics hacking in http://www.romhacking.net/forum/index.php/topic,14708.0.html
People do update sports games to reflect newer years/seasons/series/leagues/whatever but it is more involved than some initially imagine -- some imagine it is just like updating a spreadsheet but with binary/hex instead and while some of it might be like that the whole thing involves a bit more nuance at times. I don't know if it is a good absolute beginner hack, I can certainly think of gentle introductions, but I can't say it would be a bad one either if it is going to be the thing that motivates you.