You can access and change the information provided by your customers through their booking reference number (formed of 5 random letters and numbers, e.g., Y7XX4) in the Bookings tab. You can add this under the Search box and then select the blue Search button.
Once you have completed the necessary search fields to find your booking, select the booking reference number.
The next screen shows relevant information about the booking, including details for all attendees of the booking. Select Edit details next to the attendee's name.
You can then simply change their information from here.
If you're looking to fix an attendee's email address, please see here.