Change Plan price
In this help doc:
Change Plan price for all members
If you want to change a Plan's price for all members, navigate to the Plans tab, select a Plan, and click Plan Settings. Once there, update the price and save your changes. Both new and existing members will pay the new price when they sign up or at the next renewal.
Change Plan price for new Plan members
If you want to change the price for new members and keep the original price for current members, create a new Plan at the new price and replace the Plan purchase link on your site.
Then uncheck Available for purchase in the Plan Settings of the old Plan.