Kaiser

Use the old launcher or magic launcher.

I believe you can have interchangeable .jars after the new update. I may be incorrect, mind you.

What Matt said, you have two launchers: 1.5 and 1.6 That is unless you are wanting to go from 1.6.1 to 1.6? In that case, go to this page. http://www.minecraft12.com/how-to-downgrade-to-minecraft/

Follow the instructions, but instead use a 1.6 jar file, I found a few sites for the 1.6 jar download but I'm not sure which ones you'd rather use, so you might want to google for the file yourself.