When you first signed up for your Instaproofs account you may have selected a default lab in the initial setup process. However, if you'd like to change your default lab you can do that as follows:
From the main menu, click Selling » Lab Fulfillment » Lab Setup
When the page initially loads, your currently-selected default lab is selected at the top of the page.
The page is broken up into three sections, with one of them labeled Default Lab Settings. From the Default Lab Settings area, click the Adjust these Lab Settings button.
Select the lab you'd like to use as your default lab.
Click the Save button to save your new default lab.
Note: Once you've selected a default lab, be sure to go through the Lab Fulfillment Default Products settings and associate a product with each of the print options you're offering through your storefront.