In the left bar look for something called "clock frequencies" it'll pull up a menu that looks like this:

From here you're going to need to enable overclocking by clicking where it says Manual Overclocking. Immediately another screen will popup. This new screen is a basic terms of using this tool and general disclaimer. You really should read this over to understand some of the problems doing this may cause. You MUST scroll the bar down to signify you've read it. After you accept these terms you'll be able to do some frequency adjustments.

Now that you have the adjustments available to you you shouldn't ever increase things more than 2 or 3Mhz at a time. To increase your speed I suggest using the "test new settings" button instead of just applying. After you know your card you can hit apply, but please don't do it till after you test the settings! Most games are mainly limited by memory bandwidth. You'll see the biggest improvement by increasing your memory speed. It's mainly synthetic benchmarks that show "big" improvements from increasing your core speeds. If you actually put much weight in benchmarks like 3DMark the core is important to you.

Never check on apply settings at startup until you've confirmed that games play OK.

You know you've overclocked too far if you see ANY image artifacts, spots in games, basically if you see anything abnormal after overclocking then back it down slightly until the problem goes away.

As with all overclocking this will increase your heat production. In the winter you may get a lot better results than in the summer. I suggest adding a decent amount of extra cooling to your case and the card itself to help dissipate the added heat.