Depends on what you know about you know about the one who is throwing it. If they throw it on the same side. Create space by moving your pelvis to wards the corner behind you on the opposite side that they threw the knee from. Keeping your ankle relaxed to keep ground control. I throw a knee shield up. It gives me the option to kick ,leg sweep or stomp down for a lift. I personally prefer to leg sweep it lets me attack and it gets my leg back towards the ground.
If they throw on both sides youll .need to figure out what makes them throw on a side then its the same principles as before. Create space, maintain ground control, set up counter.
Or you can also throw a knee. Aim for the hip of the grounded leg.