From the Adalo Editor - Click the user icon in the top right, in the window that opens, click Teams & billing tab near the top. Then choose Change Plan
and set your plan to the free plan. That should do it.
NOTE: I would make sure you download any database data you don’t want to loose before doing this. No idea how the Adalo backend handles that when downgrading to the free plan from a paid plan. So better to be safe than sorry