You have to hit probably the delete key during startup to enter the BIOS. Then you have to change the FSB clock speed from 200 to something higher.
The FSB clock speed times the CPU multiplier = the clock speed of your CPU.
The same FSB clock speed times the RAM speed multiplier = the clock speed of your RAM, I don't recommend overclocking RAM, most RAM have very little headroom so you can down the multiplier.
Then there is CPU voltage. A higher CPU voltage can increase stability of the transistors on the CPU but at the same time too much voltage can damage the CPU. You can't killa processor by raising the clock, but you can kill a processor by raising voltage.
Heat can kill a processor as well. Read a guide to see what is the maximum safe level of voltage and look at your temps.
You may not even be able to overclock. Most pre-assembled computers have a locked BIOS that prevents overclocking. I once had an AMD 3500+ which I overclocked to 2.5Ghz from the stock 2.3 so maybe you can do the same. I forgot the settings because it was so long ago.