My guest account is the admin. I can't change it back. I use the built-in administrator but it says "error 5 occurred" " Access Denied".

How can i change back to normal?

I have two accounts, one is for me and the other is from my sister. She don't want to reset back completely because she has many files that she don't wanna lose.

How I will fix it w/o losing the files? Please help me and I hope I'll get a respond to this matter.