UPDATE! Currently, there are special enrollment periods if you want to change your Medicare plans. Between 01/01/20 - 03/31/20 you can make ONE change to your Medicare Advantage plan. It's like if you looked over your plan but disliked it, or if your doctor dropped the insurer, you can make a single change for the rest of the year. If you have only Part-A and you wish to join Part-B, and you're past your initial enrollment period, this enrollment period is also for it, but Part-B won't be effective until July. If you have Medicare Supplement, you can change, but you're subject to underwriting, and you're liable to lose pre-existing conditions. If you have Medicare PFFS and a Part-D plan, you are simply out of luck - unless you qualify for another special enrollment period.
As I mentioned before, if you're turning 65 this year, you have 3 months before your birth month, your birth month, and 3 months after your birth month, to sign up. Everyone should sign up for Medicare Part-A at 65, Part-B and Part-D can wait if you have Medicare-compliant group health insurance. There are plenty of other Special Enrollment Periods, if you think you may qualify, talk to an agent. I work in Florida, but I may be able to refer you to a local agent in your state, for enrollment questions or setting you up. If you live in Massachusetts, I am eligible to become an agent, but because it's expensive to become an agent up there, I would ask for a guaranteed sale (don't ask for me to become an agent, then go to another agent to sign up).
TS