Ensure you click Edit to make changes and 
Update to save them
Click Customer screen > Delivery Details tab
You can assign an alternative delivery address:
1.Enter delivery address information for the customer if it is different to their main address.
You can enter the name and postcode only and use the postcode lookup feature to find and complete the full address.
2.Add notes to the free type box regarding the delivery address.
Use the Google icon to locate the address or plan a route to it (from your main company address).
To add more than one delivery address click the Additional Delivery Addresses button. Then click 
Add to insert extra addresses.
The delivery addresses appear as an option when you create a delivery note so you can select from a drop down list.