If you have an ACA plan and become eligible for Medicare, you should call your ACA healthcare exchange and cancel your policy.
Note that while you do not need to enroll in Medicare Part B when you become eligible, once you reach age 65 you are automatically enrolled in Medicare Part A. This means you are no longer eligible for the premium tax credit (subsidy).
For a detailed explanation of the complications you can encounter with this procedure, you'll want to watch this video courtesy of Boomer Benefits. If you prefer to read this information, there is a transcript of the video available on our website.