1. You can edit your existing products by visiting your Products page from the left sidebar menu. Hit the options toggle button (3 small dots) for the product you wish to edit, and click on "Edit" in the drop-down menu.
2. You can also edit your product while viewing its sheet. On the top right click the "Edit" option. This will take you to the product editor where you can update or add more details to your product. Do not forget to hit "Save" after you made your changes.
If you are running subscriptions, then editing a Billing Plan will not allow changing of the price and currency, due to Stripe restrictions. The recommended course of action is to create a new Billing Plan from scratch.