If you are on FreePBX 14 already, then do a normal upgrade to 15 from within FreePBX itself. 14 to 15 is an extremely minor update. It is only module updates, not an OS update. Have a snapshot ready to revert to, but this should be painless.
Prior to upgrading to 15, I recommend that you switch your asterisk version to something current.
From the command line, as root:
asterisk-version-switch
Choose 16, and make sure everything is good. Reboot, make sure it is all still good. Then do the version upgrade to 15.
The command line instructions are the same as what I recently posted for FreePBX 15 to 16, so just follow this thread:
After you are all up and running on 15, make backups, delete the old snapshot, make a new snapshot, and then upgrade to 16 if you want.