WatchOS 5.1 Beta to 5.01

I used the Apple Watch series 3.

I installed a beta version of WatchOS 5.1 on that AppleWAtch.

Because I bought Series 4 newly, I would like to give Series 3 to my friend.

I want to change the 5.1 beta version installed on Series 3 to 5.01 which is the normal version, but I do not know how to do it.

Please let me know if anyone knows how to do it.