Unfortunately, once points are redeemed the process cannot be reversed. If you already chose to redeem your points, the good news is that means it is already on the way. If you would like to wait to choose a payment option there is no rush to do so. Tremendous does expire rewards after five years since it was initially sent (barring any updates to their Terms of Service) so as long as you choose your preferred payment method within that time frame you are welcome to do so.
If you're experiencing any issue once on Tremendous please contact help@tremendous.com with your reward ID included for assistance. IF you have any question before you reach the payment selection page on Tremendous just let us know using the Contact Us tool at the top right of the page.