I am trying to edit the velocity of a midi track in the Key Editor Contoller Display. I have selected all notes and tried to drag one of the velocity bars up but only that one moves, I need all the selected notes to move at the same time and stay in their relative position to each other.
I can’t find the answer in the manual ?
So, once you have selected the velocities, you see them in a sort of “grey box background”? (try selecting just a few of them 
).
Anyways, instead of dragging one of the Velocity “bars”, go up to the top of the “grey box”, and drag it. If you find the exact center of the top of the grey box, you can even drag proportionally rather than absolute (the little Info box by the cursor should change from “Move Vertically” to “Scale Vertically” )
There are two other methods for doing this also…
a) Having selected the notes, Shift+Control (or Shift+CMD on Mac) and hover over one of the notes (not the velocity bar )… you’ll see the cursor change to a “loudspeaker”… then drag the mouse vertically.
b) Having selected the notes, go up to the Info Line, and change the value for “velocity” that you see there (it is probably the value of the first-selected note, but all selected notes will move by the same amount).
Many thanks Vic, where the heck is your first tip in the manual… very useful, although I couldn’t get it to say “Scale Vertically” ?
as a quick test, select a few notes over a short range (so that you can see the entire “grey box” without having to scroll). Then hover the mouse over the top edge of the grey box… the cursor should change to something looking like a “v” with a “^” on top of it, and the info showing “Move vertically”. As you move the cursor to the top-center of the box, it will change into an underlined “v”, with the info “Scale vertically”.
The only problem I have with that is, precisely, when the grey bow has to be scrolled, it is very difficult to find its “real” top-center, but (sorry, I forgot about this ) you will get the "scale function if you hold down the Alt key (even though the info text doesn’t change).
The same applies to editing controllers generally, not just Velocity… see pg. 721 of the Cubase 8 operation manual .
Thanks again Vic 
I used Ctl + A to select the whole part which is why I couldn’t find the center. Selecting only a small section as you recomend shows the ‘click spots’ quite well.
Thanks for the manual page number, I totally missed that - must be getting old