Use the Break menu on the instruction window as follows:

  Action           Function
  ------           --------

  Activate All     Activates any previously set
                   breakpoints.

  Deactivate All   Deactivates any previously set
                   breakpoints.

  Cancel All       Removes all breakpoints from the
                   debugger's breakpoint list and
                   from the instruction view.

  Set...           Sets an individual breakpoint.