Select any selection tool
and use the tool's options as follows:
Add To - add to existing selection border.
Subtract From - substract from existing selection border.
To
move a selection border choose use any one of these selection tools
and select in Tool's Options palette mode to
New.
Using masks is one of the best ways to improve selection quality.
Learn more...
Refine selection
Option ⌥ + Command ⌘ + R
to reset values.