Example for 1.16.5, please check the login message to see which version Cubeville is using currently:
1) In the Minecraft launcher window, click on "Installations":
2) Click on the button "New installation":
3) Then enter "Cubeville" in the "Name" field and select "release 1.16.5" under "Version":
and click on the "Create" button:
4) Now if you go back to "Play", you will be able to choose your game version left of the "Play" button:
This way you can switch back and forth and select 1.16.5 whenever you want to play on Cubeville (or any other server that has not updated yet).