There’s a lot of information, but scattered, throughout the 1.8.5 update thread here.
There’s a solution to perform a manual upgrade in this post, but there were also some problems with getting google apps working again. I vaguely remember this has since been fixed, but I’m not sure and don’t have time to read up on it now, sorry. I strongly recommend reading up on the thread before trying anything, as a better solution may have been found, or other problems may have been discovered.
If you want to change the partition layout to the ‘regular’ updated layout, then there are some instructions here on how to flash a new binary image. In this case you can use the image binary of the software version and layout that you want to end up with (the 1.8.5 image binary was not available when the post was written). Only flash binary images if you understand what the process does. If you’re unsure, I recommend contacting fairphone support.