The correct way to add more range to your network is install another AirPort router upstairs and then connect it back to the main AirPort using a wired Ethernet connection.
There is no signal loss in the Ethernet cable, so the upstairs AirPort will receive a full strength signal to broadcast upstairs.
The upside.....you will have a network designed the way an IT pro would recommend.
The downside.....you, or someone else has to run the Ethernet cable and do some drilling.
With a concrete ceiling, it would not be recommended to try to extend the network by connecting the upstairs AirPort to the downstairs AirPort using a wireless connection. The reason is simple.......the signal will be weak and slow by the time that it gets upstairs.....and the upstairs AirPort cannot make the signal that it receives go any faster.
The upstairs AirPort acts like a signal repeater.when it connects using wireless. It can make the weak and slow signal go further, but it cannot make the signal that it receives go any faster.
If you have the time and willingness to experiment, it is possible that adding an AirPort upstairs to connect using wireless to the AirPort downstairs might provide some improvement to your current situation. But, it would also be a good idea to understand the store's return policy before you buy.
The unfortunate fact remains that wireless signals are very weak, and they will not go through concrete very well at all.