How to change a.m4a file to a.m4r file?

I just got my iPhone 5 yesterday and i wanted to get some ringtones so i tried doing it using iTunes but i don't know why i can't change the file my computer has a windows 8 system and it's not a mac (if it matters). If you know how to do it then please give an answer.

Are you high? You said the same thing!

EDIT: woops sorry i'm the one that is tripping >.> also, i don't know how

Right-click > Rename.