You should be running 7.1 to be fully compatible with vbulletin 4.15
The upgrade is easy you just unzip the download delete the following
Upload everything else left to your photopost directory on the server. That is all you need to do no upgrade script to run just updates files. This is the cleanest way to do an upgrade so you do not forget anything. You can also check out my vb
mod for vb4 to put gallery content into your forums it is pretty popular and will be included by default in our PP 8 coming out soon. That mod for the 7 series is stickied in the mod corner.