Newby question. Mtb into commuter bike

I have recently started riding my old trek 950 mtb to work. My problem is
speed.
In areas where Im really cranking slightly downhill I run out of gear. My
top gear is too slow. I believe I have normal mountain bike gears. Could
I switch the gears for smaller or faster gears. If so any ideas?

I know that I should switch to some slick tires for my 26" rims. Is there
anything else I can do to improve speed?

Thanks