To switch between different versions of the MOR, open up your Steam Library and select the 'Museum of Other Realities'

Once you're on the page, click on the Manage Icon, which will reveal a drop-down menu. From the menu, click on Properties. That should trigger a popup modal.

Click on the Betas tab and enter your access code in the 'Enter beta access code to unlock private betas' field.

Hit Check Code and, if your code is valid, you should be able to select the unlocked beta in the drop-down menu above.