From time to time, cPanel releases new themes – from X3 to Paper Lantern to Jupiter. It would’ve been nice for cPanel to build in a one-click migration wizard in WHM. Maybe, they will do so in the future.
In the meantime, here’s a command that is going to save you a ton of time from updating accounts manually.
/usr/local/cpanel/scripts/modify_accounts --theme=jupiter --all-users
The above command uses cPanel modify accounts script which has other parameters if you’re interested in customising it.