Yes! It is possible to undo the check-in.
But, following are the clause,
1. We can only perform undo check-in if the check-in operation is done from reservation stage. In short, if we take a walk-in, we can not undo the check-in.
2. The user should have the privilege to Undo check-in.
3. Check-In should have been done on same day.
If you above points match the condition follow below steps to undo the check-in.
Path: Edit transaction/room >> Options >> Undo check-in (Refer below screenshot)
Related Question: How to set Privilige for Undo Check-In for specific user, using eZee Frontdesk?