Over time you may need to change the version you play with. Here is a simple little tutorial on how to do that with both the default Minecraft downloader and launcher as well as with MagicLauncher.
First to use the Minecraft Launcher open the icon like you normally would. Once open click the Edit Profile button in the bottom left corner. In the popup window halfway down will be a drop down box. Use this to select the version you want to use. Once selected click Save Profile and then just Play.
For MagicLauncher you need to first have the version on your computer. Open the Minecraft Launcher click Play and it will download the new game version for you. Once it is on your computer you can quit the game and the Launcher and open MagicLauncher. With MagicLauncher open click Setup and look for an Environment field and use it to select the version you want to use. Click OK to save and then you can click Login to play.
A quick note with MagicLauncher, if the game is not in the environment dropdown then you can select it using the Browse button. The games jar file will most likely be in your users directory and then AppData/Roaming/.minecraft/versions/