Power down and unplug the power cord on the back.
Open the side of the case; sometimes it can be a bit tricky to find the release tabs, and there will be between 1 and 2 thumb or standard screws on the rear. Usually it is the right side (or, facing the front, your left side) panel, but it could be the other side.
The motherboard ("mobo" for short) is the (usually green) large, flat circuit board at the bottom that everything else plugs in to. On that board will be a small, silver, flat "button" about the size of a nickle-that is the CMOS battery. Holding it in is a small wire tab. Use a small screwdriver to move that wire away, or up, or whatever and the battery will pop out. Note the polarity, usually the printing on the surface faces up. With the battery out, press the power start button on the front panel and hold it for 5 seconds. Wait for 10 minutes. Now snap the battery back in (don't reverse polarity!), make sure you haven't bumped or accidentally loosened any connection cables, replace the side panel, plug in the power cable, and boot.
Let us know how it goes.
Side thought.... overclocking, especially a unit that old, is a risky thing. Those old mobos were not designed for overclocking, and I am not sure if that CPU will overclock or not. Modern computers are almost overclocked constantly and designed for it. In your computer's era they were not, and blowing a CPU by attempting an OC was not uncommon. Only the fanatic geeks attempted it.
Another side thought....if your computer clock will not hold the proper year, month and date settings it is time to replace that battery with a same type.