You can do this easily in the inspector. Make sure the midi region you want to work with is the only thing selected in the arrange window. Then in the inspector set the dynamics to 'fixed', which will make all notes play at exactly the same velocity. Then using the velocity setting right above it, you can set precisely what this fixed velocity value should be for all notes. This is a non-destructive procedure, if you want to permanently apply these changes, use 'normalize region parameters' from the midi submenu in the arrange window once you've got it sounding how you want it. This will reset the parameters in the inspector back to zero so you can make further adjustments if you want, but all the settings you made will be written directly to the midi data. When you then open the piano roll, you'll see that all your velocities are identical.
