Step 1
Locate the Order by going Sales>Orders and then hitting filter and searching for the order number including the ‘OR’:

Step 2
Press cancel on the bottom right:

Step 3
You will be prompted are you really sure you want to cancel this. If you do press the red ‘Cancel Order’ button and your customer will receive an email confirming the cancellation and their tickets will not be valid:
Step 4
You will now need to refund the customer by pressing ‘Initiate Payment’:
Depending on which payment service you’re using it will either immediately refund and you will get a confirmation or you will be directed to your payment service provider and need to complete an additional step before refunding the order on the payment service provider side.