If a voided check was written in a previous month, remove the voided check from the list of outstanding checks and write a journal entry to debit Cash and credit the account(s) that was debited when the check was originally recorded. This entry restores the cash into the checking account and eliminates the debit entered at the time the check was recorded. If the check was written in the current month, you can simply write the journal entry I just described.
Some software will allow a person to go into a previous period's activity (as well as the current period's activity) and remove the voided check or to change the amount to zero. This too will increase the cash balance and will remove the debit from the account originally debited when the check was recorded.