Select the note whose duration you want to change by double-clicking on it and then choose Duration from the Edit menu. Select the duration you want from the submenu.
If you choose Set, the Duration dialog appears. In the Duration dialog select the duration you want (the duration is in beats and steps) and then click on OK or press ENTER.
It is possible to change the duration of a group of notes so that the end time of each note is set to the start time of the next note by selecting these notes and then choosing Make Legato from the Edit menu.