Diagnosis
This isn't just a focus issue. The most common cause is that you're using keystone correction (digital picture adjustment) to try and square up the image instead of physically positioning the projector square to the wall.
How to Fix It
First, reset any digital adjustments. 1) Go into the Settings/Menu. 2) Find the Keystone or Corner Correction setting. 3) Reset it back to '0' or 'Auto'. Now, physically move your projector so it's pointing directly at your screen/wall, centered and straight-on. Use the physical focus ring on the lens only after you've got it in the right spot.
If That Didn't Work
Check if the lens itself is physically dirty. Grab a soft microfiber cloth (like for glasses) and gently wipe the glass. Don't use cleaners or press hard.
Heads Up
That little silver focus ring on the lens barrel is pretty sensitive. Make slow, small turns. Cranking it all the way in either direction won't magically fix a bad physical setup.