How do I downgrade Minecraft to play on a server running an older version?

Solution 1:

With the new launcher (launcher version 1.2.5), it's really easy to decide what version of Minecraft you want to use. Let's say I'm playing on a server that for some reason is still running Minecraft Beta 1.7.3, for that old Minecraft nostalgic feel.

In the new launcher, you're going to want to select "New Profile". In this case, we want the "Allow old beta" checked. Normally, you won't want this, but in this random case, you will. Normally, the dropdown will only show release versions; if you want to be allowed to select an older or snapshot version, you will need to check the appropriate box.

And if you want to change this version later, you can select "Edit Profile" and change it manually. Note that if you have it set to "use latest version", it will always use the latest release version (or snapshot, if that option is selected).

Solution 2:

Caution - Notch doesn't want you to do this!

You'll need to source the files for the previous version yourself and place them in .minecraft/bin, and tell your firewall to prevent Minecraft from accessing the internet so it can't download the new files. Try and find a friend who hasn't downloaded the update yet :-)