@AlanXp If you don't have access to another Macbook with non-beta Xcode installed, like me, then I guess you only have two options:
Boot MacOS from an external SSD, with Xcode installed, like @fbronner suggested.
Use Xcode Cloud.
I found Xcode Cloud to be a simpler solution in my case. I was able to set it up rather quickly. I wanted to start using it for some time and this was a good moment to do it. The free trier is largely sufficient for me.
Still, I really think they should make this limitation more explicit when updating to beta.
PS: If you are having issue with Xcode 16 beta, note that you can still run Xcode 15 from the terminal usingopen /Applications/Xcode.app/Contents/MacOS/Xcode. This won't allow you to archive the app for release (you get an error later in the process), but it may be useful for testing.