Thanks for your answer. The idea was not to change the indication Km/h into m/s, but the data info from my Weather Station and get the real km/h data.
But I understood from Adrian that that will be possible after a next update.
“Btw, it should have been better to post the question in the appropriate [thread]”, Sorry for that.
For one reason or another I could not post my question in this tread.
After changing several times between km/h and m/s (Homey App → More → Apps → Weather Underground → Configure app → Units: Wind speed (choose m/s) → Save changes - and the same for Km/h), the data in the Homey app is now (again) in real Km/h.
So, I am happy.
Sorry for all the fuss.