I highly suggest two high powered routers either from ASUS or D-Link, set one up in AP mode, and run a Cat6 cable from the first floor router to the second floor. Have both routers setup with the same SSID, and try to locate both routers in the center of the home.
I hope that our members have provided you helpful advice! I know some of them use extenders and can be a great source of information for your situation. Please let me know if we can help in any other way!