When making a selection, sometimes you grab areas you don’t want. When this happens, instead of redoing your selection, use Remove.
Removing sections from a selection in Sketchbook Pro for desktop
Once you’ve made a selection, tap 
to switch the selection tool to Remove. Then either tap-drag to remove sections from it or for the Magic Wand selection, tap areas you want deselected.
Removing sections from a selection in Sketchbook for mobile devices
Once you made a selection, tap to switch the selection tool to Remove. Then either tap-drag to remove sections from it or for Magic Wand selection, tap areas you want deselected.
