Only the owner of a file or root may change the permissions on a file no matter what its current permissions maybe.

Root may delete any file. But for any other id there are restrictions as follows: You may delete any file in any directory in which you have write access to even if you do not own the file (unless the sticky bit is set on that directory).

If you have write access to the file, but not the directory it is in, then you may edit it but NOT delete it.