Rename Extensions

I don’t know if this is the proper category for this question, but I need to know how to rename a .txt ectension into something like .html or .xml without any complexity… is it possible? I don’t want to download any program or use the DOS emulator of windows XP. the thing is… I remember years ago being able to rename extensions by right clicking and just putting rename and the extension was included there… maybe it is an option?

Well, I hope someone is able to guide me through this because until now my only solution is to open dreamweaver, put the files on a site and rename them… and that’s a long way.

THX in advance!