Select the notes, click in the Duration field in the Event Inspector (add toolbar module if it's not already showing), and enter one of:

1. A fixed amount that will be added/subtracted to each event by entering a value in ticks or M:B:T format with a "+" or "-" in front of it.
2. A single value in ticks or M:B:T format to which all events will be set.
3. A  percentage (with "%" symbol) by which you would like to increase or decrease existing durations proportionally.