Adjusting volume of one single midi note

You can raise the velocity of them, which might raise the volume, depending on the instrument and/or you can use volume curves to raise the volume of any part of a region. If they are playing at the same time as another note you don't want to raise, then the velocity is a better choice to try, then if that doesn't do it, the duping of the track and putting them on separate tracks would be better.

Select the region that has the note in it and click the "Scissor" Icon. In the Piano Roll view that opens up, select the note that you wish to be louder. To the left you'll see a slider labeled "Velocity". Drag that towards the right to make that note louder.

It should be noted that velocity doesn't do the same thing to all instruments. Mostly instruments where you can play harder to make it louder, like a drum or a bass guitar or piano, but I have noticed with a trumpet, full velocity gets you a sharp stab with a slide down of the note, and on organ it really does nothing at all. So, it depends on the instrument being edited whether that will do what you need.