This allows you to replace all occurances of an expression with another
expression. Select the term to replace and click on Substitute. The
requester that appears allows you to enter a new expression from the
keyboard or to retrieve one from the  storyboard.

While this CAN be used to replace expressions they must be of
EXACTLY the same form as the origional. If you substitute 'u' for 'x+1'
don't expect the program to replace 'x' with 'u-1'. To perform this
form of substitution you should replace 'x' with 'u-1' then call
SIMPLIFY to remove any excess constants.