How do you fix a 2011 Impala bad mph light?

Google speedometer repair. There are companies that repair clusters. You send it out and they repair and return with good turn around time, usually within a day or two.

If you know what your doing you can resolver the connections, but if you have never done this type of cluster then I suggest you take it to someone who has, most likely an independent shop and not the dealer, the dealer will try to sell you a new cluster for a whole bunch of money.