Here is a short demo showing you how to void or delete check in QuickBooks Online.  By short I mean under 1 minute.

Voiding a check only changes the amount to zero, but keeps the empty transaction in QuickBooks (other information is unchanged, for example the name of the payee, address, and date). A voided check also displays VOID in the memo field.

Deleting a check removes the transaction from QuickBooks and can’t be undone.