Each SKU exists across all your warehouses, but can only have (and must have) one preferred location.
The SKU will pull from this location if possible, at the location's cost for the SKU.
You can update this value via Products > Product Catalog Manager > Product Editor > Inventory.
You can also change this via Import/Export, using the Product Import's Preferred Location column (listing the name of the warehouse), or under the Product Warehouse Import's Is Preferred Location column (yes/no). Please see the Import legends for these imports for more details.