Diagnosis
Thermal drift. As the projector heats up, the lens assembly expands slightly, throwing off the focus, especially at the edges.
How to Fix It
Let the projector run for 20 minutes on the content you normally watch. Then, use focus ring on the lens to re-sharpen the image. You may need to do this once per viewing session after it reaches operating temperature.
If That Didn't Work
If the blur is severe and only on one side, the projector might not be level, putting the lens at an extreme angle. Physically adjust the tilt of the stand or surface so the projector is as level as possible, then re-focus.
Heads Up
This is a common physical trait of compact, single-focus-lens projectors. It's not a defect, just a characteristic. The auto-focus feature often can't compensate for it once things heat up.