If you’d like to change your plan, just go to the Explore page and pick the new plan that you want to. You can then go through the regular flow and schedule it in. When you confirm scheduling your new plan however, your current plan will be wiped and your new plan will appear in the calendar.
Each plan is built independently - therefore, you cannot start a new plan halfway through. Therefore, if you want to switch from one plan to another, you cannot pick up on the second plan where you stopped on the first one.