To edit velocities of multiple notes on the timeline, in the Edit - mode, do a rubberband selection tp select adjucent notes or press - Ctrl to select non-adjucent notes (Shift will select + mode, do a rubberband selection to select adjacent notes or press + Ctrl to select non-adjacent notes (Shift will select entire chords), then hover one of the selected notes and rotate the mouse wheel up or down. Velocities will be adjusted in all selected notes relatively, i.e. by the same amount.