Hi Martin,
Welcome to our community! 
If the game object in the Hierarchy is linked with a prefab and if you change something in that game object, you could click on the “Overrides” button in the Inspector of the game object, then on “Apply All” to apply all changes to the prefab.
You can make changes in the prefab itself. Double click on the prefab to enter the prefab mode.
Is this what you wanted to know?
See also:
- Forum User Guides : How to mark a topic as solved