Whether you can edit a plan depends on whether you already have members:
If no one has purchased a plan you can change or delete it.
If someone has purchased a plan, you can no longer change or delete it. Instead, you can archive a particular plan and create a new one to replace it. Archived plans will no longer be publicly visible. For members who have paid for archived plans, their memberships will continue to run. They'll keep paying their fixed price, and you must continue delivering the promised benefits.