You can go to the Explore page, tap on a workout plan and then go through the flow to confirm it. If it is a plan that has not yet released (and has a countdown), you can ‘secure your spot’ and you will be notified when the plan goes live. If it’s a plan that is live, or has been live in the past, then you can go ahead and schedule it into your calendar.
When you schedule a plan into your calendar, if you have an existing plan, then it will be cleared. You can only schedule one plan at a time. Once you have scheduled in a plan, you can find details about the plan in the Workout Plan Panel at the top of your dashboard.