To refund one or more donations, you must do so from your Stripe or Paypal account (whichever you attached to your campaign). Unfortunately, refunding will cost a little bit of money, depending on how many refunds you need to process. Stripe and Paypal will hold onto part of their fees.
If you decide to post-pone an event, you have the option to apply donations from this campaign as offline contributions to show the total on the new campaign when you launch it. In this scenario, best practice is to reach out and offer the donor(s) a refund. Only process the refund(s) for those who ask for it. Some people are just fine to leave the funds with the cause.