Hi how to cancel a transaction

Cancel? That’s never really an option with a transaction/purchase.

You can return the item and request a refund from the merchant.

You can contact the bank that issued the payment source and file a dispute.

Refunds generally take a few days to a couple of weeks at most. Dispute can take months to resolve.

Assuming that you mean Apple Cash, navigate to the iMessage that you sent and long-press on the amount. If the funds have not been accepted, you can cancel the transaction.