The Replace function allows you to go back and change any note once you have entered a sequence.
Clicking on the Replace button will cause a blue frame to appear around the last entered note. You can then use the up/down cursor keys to change the pitch of this note. The new fingerings will be checked immediately.
To move the pitch up or down an octave, hold down Alt while pressing the relevant cursor key. To undo your changes, press Control+Z (NB not Command+Z). To move to a different note, use the left/right cursor keys. To exit Replace mode, press Escape, or click on the Replace button again.