Honestly, it's really difficult and I wouldn't recommend it. For one thing, you can't really predict how much or in what direction it will shrink. Also, some fibers just won't shrink without felting.
It's a better bet to have it tailored, but make sure the tailor knows how to deal with knits.
It is possible to use blocking to reshape garments that have gotten stretched out with wear, but if it started out too big it will be hard to block it smaller.