The app will notify you Steam must restart. This will open a new window read the short explanation about beta participation, below is a drop-down menu with the options NONE-OPT out of all beta programs or Steam Beta Update, select drop-down menu and Choose Steam Beta Update, Click OK. On the right side your account information and options for account security, change password, and change email are displayed, below these options you will see the option Beta participation: Select the box that says CHANGE.

The Preferences window opens to Account tab selected from the sidebar on left by default. Open the Steam application, select from the menu bar at the top, Steam->Preferences. (IF YOU JUST NEED TO KNOW THE STEPS WITHOUT ALL THE DETAILED DIRECTIONS SCROLL TO BOTTOM OF THIS REPLY) I hope this is helpful. I detailed below how I resolved my issue step by step for those who prefer a detailed explanation. When I opened Steam after installing Mac OS Big Sur Beta for the public I couldn't play my game.

Ck2 has stopped working