Watch bands come in many styles today, from leather to plastic, from rubber to titanium. While many can adjusted simply, many rubber (and some leather) bands that use deployant clasps require cutting the band permanently to adjust. This can be a stressful procedure, but with conservative cutting and a careful attitude, it can be simple.
First, remove the clasp. Find the spring bars on the edges of the clasp--they are spring loaded pins that go through the band to hold it to the clasp. Push the pin down from one side and pull the band gently away from the clasp. This should release the clasp from that side of the band. Repeat for both sides.
To shorten the bracelet, cut along the marked sections (usually grooves) on the inner surface of the bracelet. If you are unsure of how much to remove, cut the pieces one at a time, then test after each cut. Do not shorten the bracelet too much.
When the bracelet is correctly cut, replace the buckle. To do this, place the spring bars in the new holes in the bracelet, then attach one side of the clasp. Push the spring bar in on one side, then pull the clasp over it to reattach. Repeat this for both sides of the clasp.
Cut less, not more--it is better to have to repeat the process more than once, as cutting it too short cannot be fixed.