Your agency address details will have been provided during the initial account setup or the first time you logged in, and this address will have been set for the "head office" agency branch. The address details will be used on statements or invoices, depending on your branding preferences.
Don't worry if you haven't got an official "head office", or indeed if you have only one office. Some agencies will have multiple branches and within ShowHouse we designate the first (and possibly only) branch to be the primary branch.
In order to update the agency address details you need to update the appropriate branch address.