Undo

  The Undo menu item lets you erase all or portions of the
  picture.

  Choose Undo to erase all moves made under your current
  tool selection.

  Note that Undo is followed by the name of the tool you are
  using.  For example, the Undo Line menu item appears when
  you select the Line tool.

  Additional topics:

  *   Using Menus and Controls

  *   Edit Menu

  *   Overview of Paint