SI unit of distance is a meter according to the International System of Units.
Conversion
We know that
1 km = 1000m
1 h = 3600 s
So 1km /h = 1000/3600 = 5/18 = 0.28 m/sec
So to convert km/h to m/s we multiply the given number by 5/18 or 0.28m/sec
Example
Speed of bicycle = 90 km/hr
To convert the speed into m/sec, we multiply by 5/18
Therefore, speed of bicycle = 90 × 5/18 m/sec = 25 m/sec
check out the video for more information on physical quantities, measurements, and units

Further reading