Rainfall units conversion from depth to rate/intensity
Your 6-hour rainfall data represent the depth of water fallen in 6 hours. In order to have the average rainfall intensity in m/s corresponding to that interval of time, you have to divide the rainfall depth for the number of seconds in the time interval or for mm/s you multiply for 1000/21600. If you want the intensity in m/h you divide for 6 hours.
You have only the cumulative rainfall in 6 hours so you can estimate the intensity dividing equally the volume of rainfall over the 6 hours. The information remains the same. The real intensity was not constant over the 6 hours, obviously. You cannot, therefore, obtain the real 1-hour rainfall depth for each hours from 6-hours rainfall depth but only an average value that assumes it constant in each hour. If you have the measured 1-hour rainfall depth data in meter and you calculate the intensity in m/s from that data the results will be different.
I hope to have correctly understood your doubts and that this made things clearer.