I tried changing outfits by pressing f6, but when i get to the last outfit, it wont change back. what should i do?

Well, uhhhh, your only choice is to get used to seeing your villager(s) wearing that last outfit! It's a one-way trip through the rotation.

i have tried changing outfits pressing F6. is there something else i should do? It didnt work for me

You have to change them in the main village screen, not the details screen. Or it could be that you're using the kind of keyboard that needs to have the F-Lock key turned on before you can access the function keys (assuming that you're not using a Mac)

You also have to have the villager selected. Meaning you click on the villager you want to change, and then press F6.

Weirdly the F6 trick also works on the GC...I got all the way to the last outfit--a pretty boring combo of white shirt and black pants--before I discovered I couldn't change him back! Oopsie. Guess he's the "BWC" now.