I know that Apple posted that you can't pair a watch with a beta earlier than 2, but I didn't see this when I updated my phone to iOS 13 beta 6. My watch wasn't syncing with my phone this morning after the update so I did a factory reset on it and now I can't sync it with any phone, including one running beta 6 as well as one on 12.4. Any ideas or should I ask the Genius Bar to bring it down to an older OS? Currently my phone is on beta 6 and my watch is probably on beta 1 or whatever happens when you do a reset. Thanks!
It can be easily solved in some basic steps without restoring anything neither watch or iDevice (Sorry for my simple English)
1. Download iOS 13 beta 5 from the internet (just type in google and you sure find something)
2. Choose your phone on the website and wait for download
3. Connect your phone and open iTunes
4. Once you are in your iDevice menu hit the update button BUT hold alt (In case of Windows CTRL) and then click it.
5. Choose downloaded file of iOS.
6. Wait for update to complete
7. Connect to your Apple watch and update them to newest software
And Voila! You're done and fully functional
Reason why is this still working is because Apple is still signing older betas and that's why you can still install it on your iPhone.
Have a nice day!