Use 'countermand' when someone with authority cancels an action or rule made previously.

Authority Required

'Countermand' is formal, often found in legal, military, or business contexts, not common in casual conversation.

Not Informal

Using 'countermand' suggests there were previous instructions that are now being changed or nullified.

Implies Conflicting Orders

 a ship with orders to carry out the first resolution was dispatched the previous day. And so another ship quickly sets sail to countermand the order – a race of democracy against time.