Sounds like a smart collection.
Just edit the collection rules as required.
You might also be talking about the default "all" collection that everything is dumped into. If you need to control what this contains, make a new collection with the handle of "all" and that will be used instead.
1. Create a new manual collection (don't add any products yet).
2. Go to the products page and set a filter to only show products from that automatic collection that you want to change.
3. Select All Products and Add to your new collection
You can delete the old automatic collection because now you have the same list of products in a manual collection