How
to update Koma-Mail:
In general you can just replace the koma_mail.exe and the languages folder with the new ones. But always make a backup of your whole Koma-Mail folder before you upgrade! And please check the changelog first, if there is anything else to consider.
Here the changelog, the newest version is at the end of the page. There is also an upgrade info if you cannot upgrade as described above.
http://www.koma-code.de/index.php?option=com_content&task=view&id=53&Itemid=6