.rar files are .zip files compressed with a different, slightly more efficient algorithm. They can be decompressed using WinRar or newer versions of WinZip, among others.
If either of those programs is installed, you should be able to open the .rar file as if it were a zip, by double clicking it, or by opening it from within WinRar or WinZip.
There's usually a readme within the rar file with instructions for how to install the patch. You'll want to read it.
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