This article explains how to add a different bill to contact or change the existing bill to contact on a Sales Order
When creating a Sales Order, you have the option to add a different Bill To contact. 
If you forget to do this when the SO is created or need to change the Bill To contact later, you can do so by editing the Sales Info from the SO Overview page.
1. Edit the Sales Info
2. From the edit window, you can 1) change the bill to 2) turn the bill to off (this will make the customer the bill to contact 3) turn the bill to on (if you didn't do this when the SO was created)
3. If you need to change the Bill To after the SO has been invoice but not paid, cancel the invoice and edit the Sales Info
4. Once the Invoice is paid, the Bill To can't be changed.