The amount of force of wind it will take to knock someone over will depend on the weight of the person.
But for a person who weighs around 100 pounds the wind would have to be around 40 to 45 miles per hour or even the force of a tropical storm to blow the person around and knock the person over.
If the person weighs more than 100 pounds then it may take 60 to 90 MPH wind force to knock someone over.