It looks like it's installed from upstream so check the PHPList site on instructions to update.

I suggest that you test it on a vanilla VM first to ensure that nothing major seriously breaks. Document your steps as you go (unless of course you are following someone else's instructions - just note anything that you do different).

Then once you are happy with that, do a backup of your server (TKLBAM is good and/or do a manual backup of the directory and a dump of the DB) and do tha same steps on your real server...