After opening Sumopaint X, you'll get a menu, where you can choose templates, already made drawings by the Sumo team which you can color, or to continue with your Cloud work.
From the menu, if you take a look next to the Sumopaint X logo, you should see Go to previous version and voilà! '
When you feel like going back to the new Sumopaint X, go to Home and re-open Sumopaint X.
note: here's a link to the previous version of Sumopaint https://sumopaint.sumo.app/?lang=en