If a customer changes tables, you can transfer the customer to the corresponding table. In general you can do it similar to this GIF:
First select the table the customers left.
Press the "Split" button. The transfer screen will appear.
In the view of your room (top right), select the table on which you want to make the rebooking.
Select the items you want to book to the table. Touch the items from the left column to move them to the middle column. All items in the column in the middle will be moved to the other table.
Now tap the "Split" button to book the items on the other table.