Problem with iCloud

I have a problem with the iCloud on my device. My uncle gave me an iPhone 5s, but he forgot to turn off the Find My iPhone and fully erase the phone. So now I have an iCloud that was used by him, it's signed out, but I can't put another account. I tried to return the password somehow and it says that the account is unavailable (it doesn't exists anymore). So I'm looking for your help, if I can remove that account somehow and put mine. Thank you :)