When a floor wears down through the finish, even through the stain and to bare wood you have a serious refinishing problem.
Bernadette wants to re-stain the warn spots without having to sand down the whole floor.
Well, the bad news is that you probably won't be able to do a good job. Sand down the finish around the warn spot and try carefully applying stain to the warn wood only, using a much lighter stain that the final colour you want. This will permit you to try several coats, rather than just a single shot at success.
The problem is wherever you put new stain over old stain, it will come out darker than over the bare floor, leaving a ring around your warn spot -- something we call a "lap mark". The good news is that you can try, and if you don't succeed, you don't have to work any harder at sanding the floor down than if you hadn't tried.
The only way to really get a perfectly even colour on your floor is to take the finish down so that the entire floor looks the same, and then coat the entire floor with stain.