This is Dallas, one of the Social Care Gurus here at Shopify.
If you are looking to edit your customers then you would want to do that in the Shopify App rather than the Shopify POS App. The POS app is more specifically designed to take orders and capture payments, whereas on the Shopify app you are able to deal with your stores back end. If you are at a farmers market or in your brick & mortar store then the POS app would be exactly what you want to use. However, any of the nitty and gritty things to do with your store would be best suited for the Shopify app. The Shopify app acts as the mobile version of the store admin that you see when you log in on the computer so it has a lot more added functionality compared to the POS.
If you would like to adjust customer settings on the Shopify app you will need to go into the online store and select Store > Customers.
When you are on the 'Customer' page you will see a list of all of the customers that have either made an account, signed up for a newsletter, or made a purchase. To edit the customer information just need to click on the customer name and then click on the pencil in the top right-hand corner.
On the customer profile, you are also able to see all the details about the customer's activity in your store. This can come in handy at times.
I hope this helps! If you have any other questions at all please don't hesitate to let me know or to call or chat in.