to
reverse an action performed during the
modeling process back to the last time the model was saved. If the Undo command is used for too many steps, the steps
will begin to be "redone."Use the Edit menu > Undo
command or the Undo button to
reverse an action performed during the
modeling process back to the last time the model was saved. If the Undo command is used for too many steps, the steps
will begin to be "redone."
Use the Edit menu > Redo
command or the Redo button 
to
restore an action performed during the modeling process.
The Undo and Redo commands work sequentially. In other words, if you have just finished the sixteenth operation since your last save, you can use the Undo feature to undo the sixteenth operation, then the fifteenth operation, and so on. You could not, however, decide that you wanted to undo only the seventh operation, for example.
The Undo and Redo commands can be used for the actions performed using the Edit, Draw, and Assign menu commands.