If you have installed SquirrelMail using RPM, then you need to update using RPM instead.
Occasional security fixes are released for SquirrelMail. To apply a patch to SquirrelMail visit http://squirrelmail.org/security/ and download your patch file. Copy the patch file to the root of the SquirrelMail directory. You must then issue the command patch -p1 < name_of_file.patch. You need to do this for every instance of SquirrelMail you may have installed.
Also as a side note if you get an error try patch -p0 < name_of_file.patch instead. Using -p1 removes the leading /, if there is no leading / then it removes the leading directory. In other words a path in the patch file that looks like this squirrelmail-1.4.4.orig/functions/addressbook.php would instead be read as /functions/addressbook.php. Using -p0 does not strip the leading character. Unfortunately using -p0 or -p1 is dependent on the author of the patch.