Before doing this back up your .minecraft folder by copying it to another location refer to step 3 if you don't know how to find it.
Step 1: Download
this.
Step 2: Force Update your Minecraft.
Image 1 is found in the bottom right of your Client, click options.
Image 2 is the popup, click 'Force Update'
This will give you a clean slate for updating.
Step 3: Drop the file downloaded in 'Step 1' into you bin folder in your .minecraft folder to get there on Windows hit ctrl+r and type in %appdata% that should get you to the .minecraft folder and you will find the bin folder in that after droping the file in select 'move and replace' option when you are asked.
(It doesn't matter if it appears as a zip for you.)
For step 3 on a mac
this tutorial should show you how to find your folder
If it still didn't work repeat the steps but instead of step 2, back up your .minecraft folder then delete it and run your Minecraft Client.
