Need help replacing window lock

I don't think you'll be able to. These sashes were not designed to be removed. The manufacturer's logic was that you would have to buy a new window if anything broke. 

To remove the lock keeper, your only hope is to try to raise the bottom sash and then lower the top sash to access it. Pull it off to expose the two holes for the pins on the back of the keeper to slide in and see if you can somehow crimp the pins from the outside of the sash frame.

I've been told that this can be a difficult process. I'm sorry we cannot offer a simpler solution.