Step 1. Find your customer
Step 2. Click on the customers name. As you'll see, you have quite a few options from here. For now we'll focus on simply Editing the customers details - to continue please click on the Edit icon on the top left hand side (shown in below image)
Step 3. Clicking on edit will bring up the Booking Process, you may recognise this when creating your first test booking. Any changes you need to make to the booking itself would be the details on the left, any details specifically to do with the customer would be done on the right. Once you've made your changes, click Save, and you're done!
Short animated version of the above: