jQuery runs in the browser and doesn't have access to files on the client machine, let alone the server machine.  Forms is an ASP.Net application and it's not set up for you to just write your own code and add it to the project.  If you want to write PHP you can do that (presuming you have a PHP runtime installed on the server), but you'll need to figure out how to hook this in to the rest of Forms.  Also, having a publicly accessible page that allows anyone to rename files has the potential to be a serious security risk.

That said, why do you want to rename a file on disk?  If it's part of a Forms process the actual name of the file is unimportant since the metadata is stored elsewhere.