Go to your Table Management Panel and click on Reservations.
Check the date and find the reservation you want to edit.

Then, filter the time at the top.

Click on the table, hold for a second or two adn drag it to another table.

If the table is empty, you will be asked if you want to move the group

or add new table - click on Move group:

If you are swapping two busy tables, just drag one table to the prefered one and the groups will be swapped.