If a mistake was made during the cash register closing and an incorrect cash balance was recorded, you have the option of correcting the amount with a subsequent deposit/withdrawal. We recommend adding a corresponding comment to the transaction to ensure traceability at a later point in time, in case of a possible cash register audit. Afterwards, you can perform the cash register closing again as described here.
A subsequent cash register closing is not possible. You can find more information on how to proceed if you forgot to do this here.