You can change the length of an existing note in StaffPad by holding a note with the pencil (without moving at first) and then moving left to decrease the duration of the note or right to increase it.
You can always decrease a note's duration but depending on the situation, you will not be able to increase it. For example, if you are in 4/4 time, the middle note in this example can be made into a half note, but it will not allow you to make it larger and push the next note out into another measure.
Similarly, the last note in the first measure below in 4/4 can not be extended past a half note because the following measure contains notes.
Dragging Notes
If you want to drag a note within a measure instead of changing its duration, pull the note left or right in one motion.