Open enrollment runs from October 15 to December 7 each year. During this time you can join Medicare, switch your plan, or drop or add a plan. As long as you make these changes during this time window your coverage will begin on January 1.
Right now is the Medicare Open Enrollment Period. From now until December 7, you can sign up for a Medicare Advantage plan, drop your Medicare Advantage plan and switch to Medicare Parts A or B, Change from one Medicare Part D plan to another or drop your Part D plan altogether. But if you miss this enrollment period, you are not completely stuck with your current plan.