I have a stained concrete floor and it is a dark brown color. Can I lighten it somehow or it will require power sanding the entire floor?
You may be able to use a tint to lighten a previously stained concrete floor. If its sealed, then the sealer may need to be removed to affect the underlying stain. It may be more difficult if the sealer and stain were applied as one product in a single process.
Concrete tint information
Here is a link that might be useful: Concrete tint information