How to join iPadOS beta program

Take a look here:

https://beta.apple.com/sp/betaprogram/

Remember not to use beta software on any device that you need for daily tasks; betas are full of bugs and can render your device inoperable. And, most important: make a backup first.

to add to babowa's post.. regarding the backup you make prior to enrolling your device.. you want to archive a backup as indicated in step #1 here

https://beta.apple.com/sp/betaprogram/enroll#ipados

You can get information on joining the Beta program(s), click through this link:

https://beta.apple.com/sp/betaprogram/