data3.big contains all the kits, they look like this.
kit_XXX_X_0.rx3
XXX is the team id
X is home (0), away (1), gk (2), third (3), etc.
you need to click on data3.big then sort by name, then scroll down to the
kit files you need & export them to the desktop.
you will need the home & away kits of the team you want & the team you want to use for the kit you want.
then take those file & place them here - C
Program Files\EA Sports\FIFA 12\Game\data\sceneassets\kit.
then reopen fm12 & browse back to 12 directory & find the kit files in the folder, export the kit images you need from the rx3 files & then import them to the rx3 files you want to use.
then rename the new rx3s to the name of the rx3 you replaced & regenerate.