No need to delete the old one.
What you need to do exactly depends on what you downloaded exactly. It could be a zip file containing other files, that you should first unzip (it makes sense to make a new folder to unzip it to). And after that, it depends on what you get. Or it could be an exe file that you should run. So I can't tell without you telling more about that.
I have just bought the game Half Life 2. It has detected that my driver is outdated and that I need the newer version. I have downloaded the newer version but am not quite sure how to go about installing it. I have read somewhere on the ATI website that I first need to unistall the old driver, but am a little worried about this. I have never done something like this before and I have been told if you do something wrong, you whole pc could end up not working properly. Please can someone help me.