What matters in directing wi-fi coverage is the orientation of the antenna(s). The signal is broadcast with greatest strength and range perpendicular to the antenna. So if your wi-fi point of origin (router, etc) has external antennas, you would want to point them horizontally in order to get vertical coverage. If it has internal antennas, you'll have to determine their orientation and mount the device in whatever position is needed.
With a wi-fi extender meant to extend a horizontal range vertically, you position the antenna on the origin router so that it covers the range needed around it (i.e. on the lower floor). Most likely this would be a vertical antenna used to create the horizontal zone of coverage. Then you position the wi-fi extender antenna so that it is both horizontal (creating a vertical range) and is perpendicular (side facing) to the origin router.
For multi-antenna routers and extenders, you can get potentially higher speeds in smaller areas by orienting all antennas the same direction. This better enables "MIMO" or "Beam-forming" technologies. Or you can sacrifice some of that potential speed and increase the coverage area by orienting some of the antennas at different angles. I.e. aiming them at different spots.
Good luck!
2Depends on the type of antenna and its orientation. The common monopole antenna radiates in a torus (donut) pattern. – sawdust – 2018-08-14T00:26:38.413