First things first: download the client jar from either the original source (probably a tweet by Jeb or Notch) or find the link somewhere else reliable, like the Minecraft forum. Or, for 1.9pre4, click here. For 1.9pre5, click here. For 1.0rc2, click here.
Next, Fire up Minecraft, and go to the Texture Packs screen.
Click the button that says Open texture pack folder and you should end up with a folder open like this.
You may not even have any texture packs, but I have quite a few installed. Now, you want to up go a folder; you can do that by clicking on ‘.minecraft’.
Next, double-click on the ‘bin’ folder and open that up.
You’ll notice that in this directory, there’s a file called ‘minecraft.jar’, and you started this all off by downloading a file called minecraft.jar.
Make sure Minecraft isn’t running.
Rename (or delete) the minecraft.jar already in this directory, and put the new one in this folder. Fire up Minecraft again, and you’ll be running 1.9!
Note: there are two different jar files called minecraft.jar. One is this one that lives in the bin directory, and the other is the Java based launcher that’s cross platform. They aren’t the same.
If you want to share this via a server message or such, I recommend using this link instead: http://bit.ly/mcupgrade
