Note duration is done by changing the length of an existing note. Usually, doubling the duration or cutting it in two, or adding a dot to a note. You can also create tuplets for uneven sets of notes (3, 5, or 7).
The second group of buttons to the right of the Piano Roll set or change the length of the note.
Technically these buttons act as „double or halve the note length value“ and „add or remove a dot“.
Changing note length with the keyboard: You can change the length of notes using the
[,] and [.] (comma and period) keys, which halve and double the note lengths, respectively.
When you increase the length of a note, the Notation Editor always „borrows“ from the next note in the bar/measure that is eligible. So when you have four crotchets (quarter notes) in a bar/measure and want to double the first note, the first end second crotchets will be collapsed to a single minim (half note) and the third and fourth notes stay as they are.
If the Notation Editor can’t honor the request, it does nothing. For instance, it can’t remove the dot from a note with no dot, and it can’t extend beyond the length of the bar/measure.
You can create tuplets from the keyboard by typing [Ctrl]+, [Ctrl]+ or [Ctrl]+ for triplets, quintuplets, and septuplets. You „untupletify“ a tuplet by [Ctrl]+.