Before you can edit a customer, you have to create it first. You can find out how to do this here.
To edit a customer, open the menu and click on Customers. A list of all saved customers will open. Now you have 2 possibilities to enter the editing mode of the customer. First you can "swipe" the list entries to the right and left. If you swipe to the right, you will see a menu where you click on edit or if you swipe to the left, you can delete the customer. Secondly, you can click on the customer and a detailed view will open. Click there on the Edit button.
Edit and save customers
Now that you are editing the customer, make your changes and click Save Customer.