Every customer in SynkedUP has a billing address stored on their client file. That's separate from a Property, which is a service location — the physical site where you do the work. A customer has one billing address and can have as many Properties as you need.
Billing address — where the customer is billed. It lives on the client file, and entering it is what automatically creates the customer's first Property.
Property — a job/service location. It lives under the Properties (yellow house) area for that customer.
Changing one does not change the other, so pick the one you actually mean.
To change the billing address
Open Clients & Properties from the blue sidebar and select the customer.
On the customer page, the fields are editable right there — there's no separate Edit button. Just type the new billing address over the old one (street, city, state/province, and zip/postal).
Click Save.
If you meant the service location instead
If you're trying to change the address where the work actually happens, edit the Property instead: open the customer, click the yellow house icon to open Properties, select the Property, edit its address, and Save. (See: How to Create Multiple Properties for a Customer.)