Okay, hopefully, this will help:
So you've got XGK installed. And you've downloaded a kit from jerseyland. In the zip file, you will most likely find two .fsh files, one for the home kit (jhxxxxx.fsh) and one for the away kit (jaxxxxx.fsh). Inside these fsh files are all the pieces of the uniform the game uses to render the players (front shirt, sleeve, back shirt, socks, shorts, etc.). If you're interested, you can view the contents of this file with EAGraph, Sonique FSH, or the FCDB XP Program that this thread refers to. I can't remember where to get them off the top of my head, but if you're interested, let me know. No sense for you trying to hunt them down.
Sorry, I digress. So now you have the .zip with the kits for the team you want to change. Unzip the file to any directory (maybe create one just for downloaded kits?). This will put the two kit files there. Then, start XGK and hit the Select button. Navigate to where you unzipped the kits and select a file. Then hit the Install button, and voila! The old kit should be overwritten! You most likely will repeat this install process to get both the home and away kits installed.
I apologize if you know some of this, I don't mean to be insulting, I just don't want to assume anything. Let me know if this helps or just frustrates you more.