How to downgrade iPhone to 14.1 or 14.0?

I did not want to update to betas 14.2, but my son did it since it kept giving the message. Now that 14.2 is installed, its terrible. Websites take forever to load, gaming is slower to load and go from screen to screen. Really the worst update i have seen.

How do i restore to 14.1 or 14.0, i did the backup on the phone prior to installing beta ios 14 when it was available some two months ago

[Re-Titled by Moderator]