You can edit the shipping rate for a customer at any time in two ways. The first is to directly edit it by pressing the 'Edit shipping rate' menu item from the shipping rate menu:

You'll either be able to choose a rate from your shipping profiles, or set a custom one:

Once you save, the shipping rate and total recurring price will be updated for the subscription.

You can also adjust the shipping rate by editing the shipping address of the customer. After you input the new address, you'll be given an option calculated from your shipping profiles, or the ability to set a custom rate: