well what you need to do is just go to the page that you log in at and go to options. then click force update. after that you press done. your minecraft.jar file will delete and recreate itself.don't worry about your texture packs and your wolds cuz they dont delete . also just wait until they update cuz if they dont the server gets shutdown after a wile. ok that was the awnser.

downgrade your Minecraft again.
When it says: Do you want to update Minecraft?
Press "Not now".

1.5 is currently very glitchy, and dosen't support servers unless u use the devoper craftbukkit