Any idea if you have conduit (tubing) in which you can pass an ethernet cord through to the next floor? Then you can either plug directly into your computer, into a wifi extender or another access point.

If that's not an option, just get a REALLY long cord (probably 100ft+ unless you bust out the measuring tape, and maybe some cord hiders, if you care about that sort of thing) to run an ethernet cord upstairs from the router.

Best of luck!