Within Teamwork Projects you can update your payment details at any time, directly through your site, if you have an active subscription.
If you need to change or add a tax number, you can follow this information also.
Note: Only site administrators in the owner company have access to this area.
Click your profile icon in the top right corner of your site, and select 'Subscription' from the dropdown menu:
In the Subscription area, you will see an option to update your payment details, below your plan summary:
You will then be able to put in your updated payment information, as well as contact details and any changes in billing address:
For more information see: Upgrading your Teamwork Projects plan