procedure FileRename(const FileName: String;
                     const NewFileName: String)

 The specified file names should contain absolute paths. Relative paths are permitted, but they rely on the current working directory. The current working directory is a per-process setting and is not reliable in a multi-threaded setting like the web server.

if FileExists('C:\temp\test.html') and (not FileExists('C:\temp\newtest.html')) then
   FileRename('C:\temp\test.html','C:\temp\newtest.html');