Your business details are under My Account > My company details. If you'd like to edit your company details, click on Edit company details. Once you've added your company details such as name, phone number, address etc, it will be visible on all documents.

Please note that if you edit your company details within Sales Tools > Templates, this will not reflect on your documents. It must be edited in My Account.

To learn more about what you can use custom templates for, please review this article: Creating & Editing Document Templates.