This game is odd.The EXP value is not in save but in gamertag. so you must mod gamertag first.
First off all,BACKUP you gamertag.
Way to mod
1.Take your gamertag from your xbox360 to PC.
2.Use Hex open the gamertag.
3.search the value E1000000 this value is too many,so you can't until you find like this.In the ASCII I red it.
so,you can see three red line in the left
First line is E1000000 you don't need mod.
Second line is your EXP
third line is your upgrade points
so,you can mod what you want. BUT BE careful The MAX upgrade potins are 34 in the game.
Credit Goes to yunasui