There are two ways, rocoat and refinish.
For Polyurethane surface finished floor, if there is no deep scratch to bare floor. You can very light sand the surface, then reapply a coat of new polyrethane. This is called recoat. This job could be DIY. You can sand it by hand or a orbital sander.
If the house is old waxed floor or it was waxed before. You can wax it.It is also very easy and DIY.
For both waxed and Polyurethane floor, You can sand it down to bare wood and reapply any surface treatment. It is called refinish. Today most people choose urethane. This is a professional job if you rent a floor sander from HD, because it is hard to control. It could be DIY if you use a hand orbital sander and do it very slow.
Now there is a chemical stripper instead of sanding, I saw it in HD for 99 dollar a kits. I heard it is between recoat and refinish.