To anyone who stumbles across this encountering the same situation, here is what I finally did to update my phone.
It was a bit of trial and error and gathering information from different threads, hope the summary helps:
- Basically following the suggestions from piero over here, variant 3: Why no updates in over a year? FP4 - #21 by piero
- I created a backup before I started, just to be sure (didn’t need it in the end, though)
- Download the update file: go to /e/OS stable FP4 download and copy the link to the latest build, but before downloading, remove the leading “IMG-” from the filename (the “IMG-” one seems to be the file for a complete installation, which is not what we want)
- Transfer the file to the SD card of your phone.
- Reboot phone into recovery mode: (on FP4: turn off, then hold vol-up + power until first vibration)
- Choose Apply Update → from external storage
- select the zip file from your SD card
- Wait for the update to finish.
- On my phone, the process seemed to be stuck in “Step 2/2” for about 90mins, so I decided to hit the back button to cancel. But the update was successful anyway
Thanks for helping, everyone. 