Lets say you bought Jarvan for 4800, you can change it for another champion thats 4800.
Or you bought vayne for 975, you can change it for corki(975)....
This is free, no cost what so ever. I don't need your password. Its totaly legal.
First press "request a trade" on the right with a note saying "guide." I will get back to you ASAP once you did it.