How to extend the wifi range?

If you get a repeater and put it half way between desired range it should work.

You have not specified where you are using the wifi and for what purpose. The range of wifi signals is of 50mtrs and if you need to amplify / increase the range then put higher dbi antenna to the router but all it will work in open space. There are other solutions also but you should provide the details of connectivity required for.

you will either need to get a repeater (which will half the throughput because of the way they're made) or you should relocate the wireless hub to the center of the house, near the ceiling. Unless your ceiling has a lot of metal or concrete in it, it should work better.

you can get a second router and put it in the first floor. this is the easiest way to do it.