Is it a .dmg file that you downloaded? If yes burn the .dmg file with disk utility, once finished restart mac and hold option key on boot and navigate to the disk, follow instructions, done. If its a package file right click, show package contents, contents folder, shared support, you can upgrade from there by clicking on the .dmg, or you can burn to disk to do fresh install. I suggest to burn to disk the .dmg even if you want to upgrade because your going to able to do a fresh install if something happens or you upgrade your HDD.
xDC