In this episode, Tunmi13 walks us through how to activate iOS and iPadOS beta updates.
To do this, first sign in with your Apple ID at either developer.apple.com, to access the developer beta, or beta.apple.com, to access the public beta. Then, once signed up for either program, go to Settings > General > Software update > Beta updates, and select the beta you signed up for. You can then go back one level in Settings, and the beta should appear like any other update that you can download and install.
Important: Beta versions of iOS and iPadOS may contain serious bugs, which is why it's generally advised, if possible, to install them on another device specifically designated for beta testing, or have a full and complete backup of your data in case a downgrade from the beta, which involves a full restore of your device, becomes necessary. For more information, check out this introduction to iOS and iPadOS beta testing.