So I've realized that a lot of people don't know how to downgrade to 1.6.4 to use some servers. With the new launcher, it's actually very easy. This works on any operating system, so don't be alarmed when you see that I'm on Ubuntu :P.

Step One: Locate the profiles section (Bottom left corner). The default profile should be your username. Click New Profile



Step Two: Name the profile and select it's version. I will be naming mine '1.6.4' and be using the 1.6.4 version of minecraft. You can name yours whatever you wish and use whatever version you want. It doesn't matter.


Step Three: Save the profile.


Step four: Select the profile from the dropdown menu.


Step Five: Click 'Play' and play minecraft :)