Look at your door lever in relation to the door. The lever itself, called the paddle, should be pointing toward the hinges of the door. If the paddle is curved, the hump of the curve should be up and the door should operate by pushing down on the handle.
Check the shaft of the lever between the handle and the round trim piece against the door, called the rose. There should be a small hole.
Either insert a small wire pin into the hole to depress the release inside the shaft or remove the small screw in the shaft with an Allen wrench of the appropriate size. The wire pin, usually with a loop at its end, comes with an interior privacy set to unlock a bed or bath privacy set from the outside in emergencies. If you no longer have it you can use the tip of a knife or other sturdy wire that fits in the hole. Levers with a set screw are usually exterior locking sets, and the set screw is on the interior side of the door.
Pull the handle off the shaft.
Repeat steps 3 and 4 with the handle on the other side.
Switch the handles, making sure they are pointing toward the hinge of the door, perpendicular to the floor.
Line up the small grooves on the inside of the lever and the outside of the shaft and slide the handle down until it clicks into place or lines up with the hole for the set screw. Tighten the set screw with the Allen wrench as applicable. Repeat on the other side.