There are separate update instructions if you have an existing installation.
Installing Noah's Classifieds is very easy - in general it's just a matter of unpacking and following the instructions of the install wizard. However, one who has never installed a web program can still experience that it is by no means so easy as installing a simple Windows program! You should first try to follow the simple instructions given here, and if you encounter problems or the instructions are not detailed enough for you, you can follow the links to read more details about the steps.
install.php in your browser and follow the instructions.