formula to convert precipitation in mm/h to runoff in m³/s
When we say there was 1 mm (or 0.001 m) of rain, it is assumed that it is on a 1 square meter area, so it is equivalent to a volume of water of 0.001 $\small\mathsf{m^3}$ (or one liter). So first you have to divide your data by 1000.

Then you have time, which is easy: there are 3600 seconds in one hour. So to convert L/h in m$\small\mathsf{^3}$/s, you have to divide your data by 3,600,000:
1 L/h = 2.7778 x 10$\small\mathsf{^{-7}}$ m$\small\mathsf{^3}$/s.

You can ask this sort of things in WolframAlpha; the trick here is to "convert" mm/h, assumed by the engine to be a speed, in L/h, the volume flow rate hidden by the mm/h unit.