What is the best way to renovate wooden floor?

Assuming the floor is sound, meaning the boards are not warped badly, there are no large gaps between boards and they are still securely attached to the sub-floor. You could apply a stripper to remove any old finish but consider that a sander with coarse paper will do the same job without any chemicals. Rent a commercial sander, the rotary kind with a means of attaching a vacuum, then using a coarse paper (80 grit) start sanding, gradually changing to finer grits until you've gotten to 220. My recommendation is to finish with three coast of water based polyurethane applied with a pad. The water based poly will not yellow like the oil based does.

Like lots of things, it's a process. First use a methylene chloride based stripper and a floor scraper. Then use oxalic acid mixed to about 15M. Rinse it and then use 50% H2O2. Rinse that and then start to sand after it dries. Go with the grain, you only go diagonally if you have a newly laid floor. Lots of sites are out there that tell you how to sand a wood floor. After that's all done, it's dealer's choice. Stain and poly is traditional, but paint or epoxy coat can be fun. Pickling can also be cool. Seriously, you can do whatever your little heart desires.

The best wayniabro jab professional so it but for simple refinishing it is entirely possible to lightly sand completely clean and then apply a coat of tinted polyurethane. Darker hues hide more imperfections. I did his in a crappy apartmen in Dallas. It helped a 1000% and cost a fraction of a pro.

Hire a belt driven floor sander and sand it down to clean wood. Then varnish it.

Its a bit difficult if you are planning it as a DIY project. You can always search for professionals in your area. Try to take some quotations so hat you can opt depending on your budget.