Processes in Microsoft 365 Insider for installing preview builds, activating subscriptions, and keeping apps up to date with the latest Insider features.
Information about opting-out of the program can be found here. Once you opt out, you need to wait until the production version catches up with the Insider build you are on to get back onto Production builds. You can also uninstall and reinstall Office without opting back in to the program, and that should put you on Production builds immediately. If doing a regular uninstall of Office isn't, for some reason, removing the Insider builds, you can do a full, manual uninstall by following the steps here.
-Jesse