You are in the editing mode of your product under "Category assignment" and you mistakenly put a product in the wrong category? This is how you undo it:
1. Click "Save" if you still have unsaved product changes.
2. Click "Categories".
3. Click on the affected Category.
4. Scroll down to the bottom to see all the products assigned to this category.
5. Delete all the unfitting products via clicking on the ''remove'' icon.
Save and you are good to go :-)