English Translation
On warm days, bears stretch out or lie on their backs with their feet in the air.
“Bears” is the subject (doer), but the subject of the Japanese sentence would be “we” (human beings) or “I”.
Besides, 足を宙に上げる is a bad collocation. We say 宙に浮く (floating in the sky), 宙吊りになる (handed in the air = cannot be down to the ground), 宙に打ち上げる (shoot a rocket to the sky) etc. we don’t say 宙に上げる (lift something from the ground to the sky).
On top of that, 寝る means “to sleep”, not “to lie”. I would propose:
lie on their backs with their feet in the air. = 仰向けに寝そべって足を上げる。