Headlights flicker?

There are other things that can cause the lights to flicker, so the place to start is by going over this guide to check the changing voltage and if no good this guide will show you how to replace the unit.

https://www.2carpros.com/articles/how-to-check-a-car-alternator

and

https://www.2carpros.com/articles/how-to-replace-an-alternator

Also, if you have a digital dash, those are very susceptible to voltage fluctuations. The dash brightness can flicker and be very irritating even before you see that in the rest of the car's lights. To partially address that, the voltage regulator senses voltage right at the dash on those cars instead of internally. That adds a lot more wire and connector terminals that can corrode.

If you have a replacement alternator already, replacing it is the fastest test.