How can I dispute a transaction

Apple Pay is just a way to use a credit or debit card. You would dispute a transaction with the card issuer, just as you would if you had actually used the card. If you mean an Apple Cash transaction, that is the same as using cash. There is no way to dispute it from Apple’s end, just like if you buy something from someone and give them cash you can’t dispute it, except with the person you gave the cash to. You should only send Apple Cash to someone you know, never to a stranger. And if you use Apple Cash to pay a business you would have to dispute it with that business.

If that isn’t what you meant please explain.