This is by far the easiest way to manually update your Windows Phone 7.5 phone to the spankin’ new 7.8. Windows Phone Hacker has released a tool called Seven-Eighter which is an executable app that streamlines CAB downloads and installations.
After updating the Dell Venue Pro to 7.8, I just updated another Windows Phone 7.5 Samsung Focus using this and while it took a while (5 updates) it was fairly easy and would just need a little amount of technical know-how.
Basically, the tool acts as a wrapper for the UpdateWP file, and contains all the updates and their language packs. Using this tool has the same risk as using the manual cab method, so keep in mind that I'm not responsible for the chance something goes wrong, despite that being very low. It's also a tad experimental, but it seemed to work for a great deal of people while I was sleeping, so have at it ;)
Just click the Install WP7.8, and remember to select the correct language packs (same as your phone) to avoid any kinks after.