I will see if I can get the mail floor extreme to extend the network.
Here is one way to do that using your Mac. If you have an iPhone or iPad handy, that device could be used as well.
Temporarily, move the "upstairs" Extreme to the basement or close proximity to the basement AirPort Extreme.
Power up the upstairs Extreme for a minute or two
Hold in the reset button on the back of the Extreme for 5 to 6 seconds and release
Allow a full minute for the Extreme to restart to a slow, blinking amber light status
On your Mac.....
Click the WiFi icon at the top of the screen
Look for a listing of New AirPort Base Station
Click directly on AirPort Extreme

If you are using an iPhone or iPad.....
Tap Settings on the home screen
Tap WiFi
Look for a listing of Set Up AirPort Base Station
Tap on AirPort Extreme
That will get the setup 'wizard" running. It will take a few seconds to analyze the network, then display a screen that looks like this....except that you will see your devices pictured and network name displayed

Enter a device that you want to call the upstairs Extreme
Click Next
The setup wizard will configure everything for you. When you see a message of Setup Complete, click Done.
Now move the Extreme to the location where you want it upstairs. The more that you minimize obstructions between the AirPorts, the better the network will operate. If you want the best performance from your network, you will need to run an Ethernet cable from the basement Extreme to the upstairs Extreme and then set up the upstairs Extreme again.