You must be a Vendor Admin or Office Admin to do this.
It will not be required by all clients.
Navigate to Menu > My Firm
Here, you can add or edit your billing details and contact details.
Click the update button after editing each section to save your details.
Please note: Until you enter billing or contact info, firm details not submitted will appear beside each user's name. You can ignore this if it's not a requirement for your client.