You just rename it normally. I assume that your problem is that you can't see the whole filename, just the part before ".rar". To fix that, in an explorer window, click the Tools menu, then Folder Options. In Folder Options, click the second tab at the top (called "view" I think). Scroll thouugh the options in the bottom half of that dialog. Find the option titled something like "Hide extensions for known file types". Untick that option. Click OK/Close. The entire filename will now be visible to you.