You can add or edit existing clients in your account by going to the Clients page, found on the left-hand hamburger menu:
You can add a client to your account by tapping the "+" button at the bottom right of the page:
From there, a menu will appear asking if you would like to add a client organization or client contact to your list:
Adding a client will bring you to the client organization page, where you can add the client organization's name and address:
Adding a client contact will produce a similar form, but will include contact details for the client contact themselves, such as phone number, email address, and additional address fields (if they aren't located at the same address as the client organization):
Once you're done, tap the Save button at the top right of the page to save your changes.
Whether you're looking to edit a client organization or client contact, you can do so by entering that client's page and tapping the pen icon at the top right of the page:
This will pull up the edit menu, where you can change or add to the client contact details.