If you were meant to pledge on a reward instead of donating and the project is still live with longer than 24 hours left, then you can claim a reward up to the value of your donation instead.
Sign in to your account and go to your pledges to manage your payments
Click on the + button on the right hand side of the donation that you want to change
You will then be able to select a reward up to the value of your donation.
You cannot swap your reward for a different reward. If you pledged on the wrong reward by mistake you will need to cancel your pledge then pledge on the correct reward.
Follow these instructions on how to cancel your payment.