You can subtract from the selection area using the selection tools, either the same selection tool you used to make the original selection or a different selection tool. You can also use the Contract command to contract a selection by a specific number of pixels.
To subtract from a selection using the selection tools:
Hold down the Ctrl key as you click the parts of the image you want to remove from the selection.
To contract the selection by a specific number of pixels:
Choose Selections > Modify > Contract.
Select the number of pixels by which you want to contract the selection.
Click OK. The marquee contracts while keeping its original shape.
Adding or Removing a Color from a Selection
Changing the Feathering of a Selection
Cleaning up the Edges of a Selection
Modifying the Select Selection Borders