To set up your new shipping rate, go to Profile and then Info and Settings. Scroll to the bottom of that page and on the left-hand side, you will see the shipping menu option. Click on that and then scroll back up. Enter your shipping rate. Note, only one shipping rate can be active at one time.
If you use Zensales, then you can also head over to My Business/Zensales and then click on Connect Zensales, and follow the process through