If you go to
www.buykawasaki.com
and enter your bikes information (not the VIN on these old bikes you can find anything related to your bike there.
I found by looking up your bike that you have adjustment bolts at the end of your rear swingarm. Put the bike on its centerstand and at the back of the swingarm there are 2 bolts. Loosen the nut onthe axle and at the end of them and tighten the bolt at the tip of it. Make sure both sides are equal with the marks on your swingarm. Some say to measure it with a tape measure also.
If Im not mistaken you should have about 1/2" of play in the belt.
I have chains on my bikes so maybe Im way off and a squeaky belt is a sign of time for a new belt. Someone else may have a better idea of this.