The file.rename( ) method renames the file with the name you passed in newName and returns the renamed File object.
The newName parameter must comply with naming rules (e.g., it must not contain characters such as ":", "/", etc.), otherwise an error is returned. If a file with the same name already exists, an error is returned.
Note that the method modifies the full name of the file, i.e. if you do not pass an extension in newName, the file will have a name without an extension.
Returned object
The method returns the renamed File object.
You want to rename "ReadMe.txt" in "ReadMe_new.txt":
$toRename:=File("C:\\Documents\\Archives\\ReadMe.txt";fk platform path)
$newName:=$toRename.rename($toRename.name+"_new"+$toRename.extension)