Take out the rear cocking rod, insert the right sized allen key and dial turn anticlockwise, re-insert cocking rod, chrono and repeat until desired velocity is set.
I have always used past sale threads to price my gear up as it depends on condition etc, just search for your marker in the classifieds you would be very unlucky not to find the marker being sold before or currently up for sale and it will give you an idea of what prices are shifting the marker and what prices are making the markers not sell. hope that helps