One should be assigned as primary if a product has more than one warehouse. If you wish to assign a different warehouse as primary, follow these steps:
Click into Inventory, Manage from the individual product screen
Click on the name of the warehouse you wish to make primary
On the next inventory detail page, click the red Make Primary button
If you need to mass update all inventory records to have their primary warehouse reset:
From the Products tab, click on Manage Warehouses
Click on the name of the warehouse you wish to set as primary
Click the Special Actions button on the left-side panel to either
Add that warehouse to all products
Set at Primary Inventory (only for products that have this warehouse/inventory)