I haven't seen any good tutorials on this, and not all of us are computer savvy, so here it goes.
BTW this is only going to work on windows, i am not at all familiar with other systems.
1) search %appdata% in the start menu
2) go to your .minecraft folder
3) go to your bin folder
4) right click on the file Minecraft(not minecraft-1.1 or minecraft-1.2.3 or minecraft-whatever) and click restore previous versions
5) its pretty self explanatory from there(the menu does a nice job of instructing you)
6) leave feedback! this is the first time that i have ever posted a tutorial, and any advice/criticism will be taken into consideration