Take the router that you want to put on the second floor, and connect a PC or laptop directly to it, and get into the web admin interface. Turn OFF DHCP on the second router, and ensure that it is using an IP Address in the same subnet as the first router. If you post up the router model info, we can make sure they are in the same range... Ex: If Router 1 is 192.168.1.1, make the second 192.168.1.2 or something like that. Then configure the router so that the wireless SSID and password are the same as the first router.
Then, connect a cable between the 2 routers. Connect them using the LAN ports on both. To make it easy to remember, I would use the SAME port on both routers so you know when one is the uplink. Use PORT 1 or PORT 4 on by devices. At this point, you should be good to go.