You need to restart SketchBook for this change to take effect. See the article below, Changing the maximum number of undos, for instructions. The default is 50 however, if you don’t foresee yourself needing to go back that many steps, you can change this value to something like 25. That might seem like a lot, and to me 50 is however, to someone doing iterative design, it might be necessary. We recommend a number within the range of 2-50. SketchBook give you the ability to set the maximum number of times you can go back and undo something within the app. You can change the number of undos in the General tab of the Preferences. To redo an action, in the toolbar or from the lagoon, tap or press Shift+Cmd+Z for Mac or Ctrl+Y for Windows.To undo the previous action, in the toolbar or from the lagoon, tap or press Cmd+Z for Mac or Ctrl+Z for Windows.The default number of undos and redos is 50 however, see Setting your maximum level of undos for more information. Undoing and redoing in SketchBook Pro Desktop The version of SketchBook you're running determines the number of levels of undo and redo available. When you place a stroke and decide it’s not what you had in mind, you can undo it. Well, with Undo and Redo, these issues don’t exist! SketchBook is more forgiving than paper or canvas. How many times have you sketched something and wished you could get back what you erased? Have you worked with something other than a pencil and wished you could erase that last stroke, undoing the mistake?

Autodesk sketchbook editing