open Minecraft and on the screen that you click "play" by the profile button on the bottom left, it says "edit profile". click on that and it brings up a tab. look down and you will see "use version" then click on the bar and it will bring up a list of versions and click on which one you want
i already downloaded forge, but when i open it and say install client, it loads for a second then says, these libraries failed to download please try to download later. im running 1.6.4 and i have a windows computer