First, you will want to navigate to the event check-in tab. Then you will want to search the ticket holder (or table), click the actions button, and select re-assign. You will need the phone number/email of the new ticket holder to successfully re-assign a ticket.
If the attendee is part of a party you will see a different, smaller drop-down menu; in this case, you will select View Party.
Next select Add Attendee where you wish to assign an attendee, shown below:
You will then be prompted to add the individual via mobile phone or email, if the attendee has an account they will be assigned the ticket:
If the attendee does not have an account, you will be prompted to create an account. After creating the account you will then be able to assign them the ticket: