There is no easy answer. Removing the Rs from the register can turn into a long, tedious process.
Unfortunately, QuickBooks Online doesn’t currently have a way written into the program to undo a reconcile all at once. If you feel the reconcile must be undo, then we do require that reconciles be undone transaction by transaction.
Ask your Accountant or a Certified ProAdvisor with QB Online Accountant (QBOA) subscription to undo it for you. They can do it in QuickBooks Online Accountant.