As long as the plan does not have any subscribed members, you can edit it.
If the plan has members, any updates you make, like changing the price, will not affect existing members.
Members previously subscribed to that plan will continue to be billed at their original sign up amount. Only new signups will be charged at the updated price.
If you want to increase the price of an existing subscription, Disable the existing subscription plan and create a new plan with the updated price.
Recurring members subscribed to the disabled plan will continue to be billed until they cancel their subscription.