I had Syncthing setup and working great between my Amazon tablet and my desktop PC for files in a vault for Obsidian.md. Then I had an issue with my primary drive (Gigabyte, no longer used) and a windows update to where I had to reinstall Windows completely. Luckily I had backups of all my important data but had completely forgotten about Obsidian on my tablet. Syncthing is still set up on my tablet but it doesn’t recognize my desktop after the reinstall. Is there a way to get Syncthing working again without having to readd all my folders and files I had it syncing before?
That depends…
Sounds to me like your easiest way forward is to add the Desktop PC as a new device to your Tablet, and then share the folder with that “new” device. That way all the files get downloaded from your tablet to the PC.
But if you had a very complicated setup in Syncthing (which I do not really assume based on your post) and your backup contains the complete user profile folder you could also try to locate and copy the configuration file and certificates from your backup to the new installation. BUT: I would not recommend copying over the database unless you know EXACTLY what you are doing.
…but it also sounds like you first need to figure out how you had installed Syncthing to your PC (is it Windows?) in the first place.
That… I cannot really give you any recommendations about without knowing how much you know about your setup 
Well, the files on the PC are newer than the ones on the tablet so backing the tablet up to the PC would not be advisable. I basically want to update the tablet with the files on my PC… Yes, it is Windows 11 pro and I’ve reinstalled Syncthing.