You can use the selection tools to add to or subtract from existing pixel selections.
Before manually adding to or subtracting from a selection, you may want to set the feather and anti-aliasing values in the options bar to the same settings used in the original selection.
Select the Add To Selection option in
the options bar, and drag to add to the selection.
Hold down Shift, and drag to add to the selection.
A plus sign appears next to the pointer when you’re adding to a selection.
Select the Subtract From Selection option in
the options bar, and drag to intersect with other selections.
Hold down Alt (Windows) or Option (Mac OS), and drag to subtract another selection.
A minus sign appears next to the pointer when you’re subtracting from a selection.
Select the Intersect With Selection option in
the options bar, and drag.
Hold down Alt+Shift (Windows) or Option+Shift (Mac OS) and drag over the portion of the original selection that you want to select.
An “x” appears next to the pointer when you’re selecting an intersected area.