Hi @Fritz782,
If you are referring to an engine-provided material, unfortunately there is no simple way to revert these changes. Theoretically, you would have to redownload the entire engine.
Luckily for you, this is a very simple material and it has existed for quite a few engine versions. I would recommend opening up a project made with a different engine version, opening that version’s material, and copy-pasting it into your working engine’s material.
Good luck!