You can edit notes in the Score Editor or the Piano Roll editor. Select the arrow tool in the toolbar (the first one on the left). If you are in the Score Editor, make sure the note is selected in the object palette. Select a note by double-clicking, control-clicking or dragging over the note with the mouse. The Edit menu will appear. Choose the editing operation you would like.
You can also erase or move notes using the other tools in the toolbar except the pencil, (which is used for entering notes). If you are in the Score Editor, make sure the note is selected in the object palette. Select the tool you want and apply the edit by clicking on the note (to erase using the eraser tool) or by dragging the note up, down, left or right.