Unlike a door knob, if door levers become loose they will fail to operate correctly. They can become loose with ordinary use, and should be corrected before the entire assembly fails.
Step 1 - Remove the Screws
You can remove the screw using either the Allen wrench or the screwdriver, depending on the type of lever you have. Take the set screw out first. This is on the bottom portion of the lever. Remove the lever handle.
Step 2 - Align it Correctly
With the handle off you should be able to see the surface on the round rod inside. With this in sight you can replace the handle and work it until it aligns with this surface properly.
Step 3 - Tighten and Replace Screws
Keep the handle in place and replace the set screw on the bottom of the lever. Tighten this with the screwdriver. Go slowly with this so you won't end up stripping the screw head. Press down on the lever to reset the lock assembly.
Step 4 - Repeat on Opposite Side
Repeat this process for the other side of the door as well. It's important that both levers are adjusted correctly so it will function.