You can change the currency of a Deal only if it does not contain any Products and is not synchronized with a Quote or an Order.
If the Deal has Products assigned, you can remove them directly on the Deal detail page. To delete a Product, the Deal must be in an open status – meaning it must not be in the Won, Lost, or Canceled status. Once this condition is met, in the Products panel click the button with three dots next to the specific Product on the right and select Remove.
You can cancel the synchronization on the detail page of the respective Quote or Order by clicking the cross icon in the panel labeled Synchronized.