You can use a hammer and large screw driver to adjust the rear shock. But I don't advise doing it just to change the seat height.
You need to setup the rear end for your weight.
Its pretty easy to do. And there are quite a few resources online to tell you exactly the steps.
It's basically measuring the rear end while the wheel is off the ground and then sitting on the bike over the pegs (normal riding position) and taking another measurement in the same place. (Rear axle to end of silencer is good)
Convert to mm if you don't have a metric ruler, and subtract the second number from the first.
That's your sag. It should be between 85 and 100 mm.
It the sag is lower than 85 you need to loosen the rear end, if its more than 100mm you need to tighten it.
Then after you get it set right, take that same measurement again with out a load on the bike.. Just the static weight of the bike. It should sag about 25 mm.
If you get all of that set right, and it's still too low for you, you should either consider a 125 or a big wheel kit for your 85.