Replacing a selection dismisses the current selected area and starts a new one. It is the default state of the selection tool. But if you've changed your selection tool state to Add or Subtract, you may need to change it to Replace.
Replacing a selection in Sketchbook Pro for desktop
In the selection sub-toolbar, make sure Replace is selected, then tap-drag to capture a new selection.
Replacing a selection in Sketchbook for mobile devices
In the selection sub-toolbar, make sure Replace is selected, then tap-drag to capture a new selection.
