Try tightening the belt a little first, but don't overdo it. If that doesn't help, just replace it. Don't put WD-40 on the belt - it's squealing because it's slipping, and that will only make it worse.

As mentioned above, you'll have to remove an engine mount to replace the belt, and you might as well replace the power steering belt while you're at it. You have to remove it to replace the A/C belt anyway. The alternator belt is also pretty straight forward if you've gotten that far. It takes me 30-45 min to replace an A/C belt, and you'll just need a basic wrench/socket set and a jack to put under the oil pan. Allocate two hours though, because some of the bolts could be a huge pain in the ass to break loose if you haven't done this in a while (or ever).