While airborne, push the control stick in any direction while tapping the Shield button. See defense.
Depending on how agile your fighter is in midair, and how much altitude you have to work with, you can also implement air dodges and directional air dodges in your offstage recovery game. Some fighters have just their up specials to work with for recovery, while many have some kind of horizontal move to go alongside their eventual up specials. Keep in mind the option of mixing in an air dodge directed up-forward to gain extra horizontal distance if it doesn’t give up too much height. Or there’s dodging up-back as a midair anti-edge-guarding feint, before you attempt to side special/up special back in as normal. But also keep in mind that this isn’t for everyone, because lots of fighters have tons of air lag after an angled air dodge.