I am trying to follow this simple tutorial https://www.youtube.com/watch?v=ZI49VIcISaw that changes the units to mm and after making the necessary modifications I’ve tried both autosaving the preferences and manually saving them but if I restart blender I always get back the old ones.
Full video below:
You can try this; File>Defaults>Save as startup.
The location where the preferences are saved is:
So first thing you could do is make sure that this folder is writable. The Blender version you’re using store the preferences in the folder named after it’s version.
BUT what you seem to try to do is change the unit of your default file, for example, to always have it be in mm or inches. For this, @digitvisions has the right answer:
This will make the active blender file the default template that Blender will open. Again, this is stored in APPDATA so if it doesn’t seem to work, make certain that this folder is writable.