When you create a new version, it synchronises content with the Parent version, by default.
You can unsync this content by editing any field in a version that's not the parent. This unsyncs the content for this version. It unsyncs on an element by element basis. For example, if you edit the content for a sub-headline, it will only unsync for the sub-headline.
If you then change the same element in the parent version, the edits won't carry through.
You can also control whether an element in a module is inheriting from the parent version or not by using the sync button
If you sync with the parent version, the content in your version will be replaced by the content from the parent version. If you unsync, it will replace it with the content that was there before.
If the parent version hasn't been edited, you can reset the content back to what was in the Email Design System.