How do you replace ac ballast where emergency ballast present?

First you need to determine if it's the ballast that's bad or the emergency ballast. The emergency ballast can go bad and cause the bulbs not to work or not all of them to work depending on the lights wiring setup. Push the test button on the emergency ballast which should have a red glowing indicator light. If all the lights illuminate slightly, change the emergency ballast. It's best to buy the exact ballast for this repair. i.e. If it's a T8 3 lamp fixture. Buy a T8 3 lamp ballast not a 4. You can use a 4 in place of a 3 lamp, but the 3 lamp will be more user friendly. Easiest way is to note what wires go to what. Draw a diagram if you have to and just simply match them back up.