if you don't know how your in luck.

STEP 1
you need to download a skin from your computer and name it char
STEP 2
you need to download Iexplorer or Ifunbox and open it
STEP 3
go to apps<minecraft<minecraft.app(and plugin first)
STEP 4
put char in the file anywhere and wait till its done loading(shows at the bottom left)
STEP 5
unplug and there you go! you got a skin!