There is no way to automatically undo a cancelled registration. You will need to re-register the participant using a discount code.

If you have cancelled a participant's registration in error and would like to undo this, here are the steps you would have to take:
  1. Create 100% discount code (From Tools menu in the event dashboard, select Discounts)
  2. Collect all of the participant's registration information from their cancelled registration
  3. Re-register the participant (From event dashboard click Register Someone)
Note: If you need to issue refund for a cancelled registration, please refer to Issue Refund for a Cancelled Registration.