I have a screw that is stuck on a metal plate. What type of filing tool will be suitable to file the screw head so that it change from round to square shape?
For a small screw, a warding file with a flat edge. For a larger screw a normal flat metal file with a flat edge.
By flat file I mean one with a rectangular cross section. These usually have cutting teeth on the larger "face" sides. The thinner edges usually have one narrow edge with teeth and one narrow edge that is a smooth "safe" edge without any teeth. You can buy these at any DIY/hardware store.
This assumes the screw is a normal unhardened steel or brass machine screw of modest size.
A fairly common way to remove stubborn, seized, corroded or damaged screws is heat, penetrating oil and a "screw extractor"