Repulsorlifts cannot work upside down as they create an anti-gravitic effect, basically repelling the local gravity in order to allow a vehicle to float. Thus they needs to be always angled towards the main source of local gravity, usually the center of the planet or moon the vehicle is standing upon.
As stated in the relevant Wookieepedia article:
Canon: Repulsorlift was a technology that allowed a craft to hover or even fly over a planet's surface by pushing against its gravity, producing thrust.
Legends: The repulsorlift or repulsorlift engine, often referred to simply as a repulsor, was an anti-gravity technology capable of levitating an object.
Repulsorlifts are the main source of lift on a planet surface, but they're not the main source of thrust: every vehicle we've seen has some sort of reactor to propel it forward, from Luke's landspeeder to the Imperial bikes used on Endor to the X-Wings. Which is how they can move and perform maneuvers.
X-Wings, for example, can fly upside down and roll much in the same way planes on our planet can do such maneuvers: because they rely on their speed and their wings to produce air lift and keep them in the air while they maneuver.
A stationary vehicle, relying only on its repulsorlift to keep it up, would probably drift to the side if it was tilted too strongly to one side, although it seems likely that those vehicles are equipped with a system equivalent to gyroscopes that help keep them in place. We can see it when Luke jumps on his landspeeder: it tilts some but doesn't seem to drift to the side, or, during the bike chase on Endor, when Leia jumps off her bike and it sways from side to side but keep flying in a straight line.