This article explains how to find a customer and open their record in the Back Office. For an explanation of every field and tab on the customer record itself, see Understanding the Customer Record in the Back Office.
1) In the Back Office side menu, expand the Customers heading and click Customers:

2) This opens your customer list. Type a search term into one of the search fields, then press Enter.
3) Your results will show in the grid below. Double-click a customer, or click the edit icon against their record, to open their customer card:

4) Use the tabs on the customer card to view their Account Information, Account Transactions, Orders & Workshop Jobs, Purchase History, Messages and Communications, Delivery Addresses, Gift Vouchers / Gift Certificates, and Serviced Items:

5) When making a change to a Customer's Account, such as their address, you'll need to click Save Pending Changes to commit the edits you’ve made:

For a full explanation of what each tab and field contains, see Understanding the Customer Record in the Back Office.