In the MIDI Editor you can have a look at using the
View > Filter events window to filter & select only specific Events, first (in your example:
Event Type: [Note] inside a certain low/high
Velocity value range).
And then adjust the velocity (of all selected Events) for instance via the
Note Properties or by dragging the velocity bars/handles with the mouse.
An example using the Filter event window is shown here:
https://mrctechy.wordpress.com/2014/...tor-in-reaper/