As shared components are reusable and shared across multiples pages and templates, it is not possible to modify them outside of the Shared Component designer.
When adding a shared component in another design tool, such as the Page designer, that shared component is an immutable reference to the original shared component.
It is possible to customize a shared component by adding a Slot component. Dropping a Slot component onto the canvas creates a dedicated area where a specific instance of a shared component can be modified. Modifying a specific instance of component has no effect on the original shared component.
Defining a clear name for the slot helps to convey the intent to all users of the component.