When making a selection, sometimes you don’t grab everything the first time. When this happens, instead of re-selecting everything, use 
Add.
Adding to a selection in Sketchbook Pro for desktop
Once you’ve made a selection, in the selection sub-toolbar tap , then either tap-drag to add to your selection or when using Magic Wand selection, tap other areas. If you are using Magic Wand, you may need to increase the Tolerance to add similar pixels to your selection.
Adding to a selection in Sketchbook for mobile devices
Once you've made a selection, in the selection sub-toolbar tap . Then either tap-drag to add to your selection or when using Magic Wand selection, tap other areas. If you using Magic Wand, you may need to increase the Tolerance to add more similar pixels to your selection.
