There's a whole section in the Docs about upgrading. http://documentation.coppermine-gallery.net/en/upgrading.htm#upgrade
As you are already using 1.5.x you need to read http://documentation.coppermine-gallery.net/en/upgrading.htm#upgrade_any
As you are already using 1.5.x you need to read http://documentation.coppermine-gallery.net/en/upgrading.htm#upgrade_any