Back before you were born, scientists used to measure the speed of light. In the front of old science books, the current measurement was printed.
And, the speed was slowly changing. Whether that was because of measurement error or the speed changing, we do not know.
THEN (dun dun dunnnnn) the scientist got together and set the speed of light. They decided that 299,792,458 m/s was the speed of light.
It hasn't been measured since then, except as some stunt to show they could still do it.