If you want, you can cut out part of what was already in a track and record something new in its place. You do this by punch recording. Click on the Punch button in the main control area (the button with the red line and two little black arrows underneath it) and click on the time display to the right of the Punch button.
Set the From time to the start of the passage you want to cut out and set the To time to the end of the passage you want to cut out. Now when you record, all events in the passage will be replaced by events recorded from the keyboard. If you don't like what you did, you can reject the take, or you can undo it after listening to it a few times.