Just updated Kstars on my PC to latest version 3.5.5. I have lost all my previous KStars settings including EKOS equipment profiles, geographic location etc. Is there a backup file from which I can re-store previous settings?

First off the latest KStars is 3.5.9 not 3.5.5.
Windows settings are stored in C:\Users\username\AppData\Local\kstars.
If you have a backup of your drive you can copy the settings from there.

Oops sorry. Typo I meant 3.5.9. I don't have a backup of this area. I did take a look at Appdata after upgrading and installed SQLite app to take a look at file userdb.sqlite.backup but this only had simulator under profile. Maybe I am not looking in the correct place? Weirdly in the log there is a line: Cannot center on "HD191939" : no object found. That is a star I searched for in the previous version of Kstars so it remembers some things?

Then you need to reconfigure everything.
Also re-download DSO catalogs. Menu/Data/Download New Data.
Back-up your drive(s) regularly!

I installed a fresh version of Windows recently and was able to restore all settings from backups. For all tricky applications including KStars.

Not criticising as developers are doing a great job but most applications report a previous version exists and will carry over previous settings adjusting where setting format has changed. Its taken me several months to get to a point where I can reliably let the telescope run unattended overnight without waking up to find it failed at some point. Anyway those obscure options to find and tick are still fresh in mind.

I never lost my settings. Not sure what happened in your case.
Settings are stored in a separate folder. When an update is installed, the settings should be safe.

Ah.. I just remembered.
The settings used to be stored in C:\Users\username\AppData\Roaming\kstars.
Now they are in C:\Users\username\AppData\Local\kstars.
Check your Roaming\kstars folder.

Ah ah. Yes that looks promising. Can see my profiles and there are two ini files for SavedFocus and Saved Align. I will back up 3.5.9 settings and experiment copying over setting files form roaming. Thanks for that.