Hello Roby!
In order to change the volumetric Clouds speed you have to acces the material instance of the clouds.
1- Open the material instance of the clouds by selecting the volumetric clouds asset on the outliner.
2- Scroll down the “Global Vector Parameter Values” in the material instance and look for “WindVector” parameter, the “R” and “G” values will change the speed in the X and Y axis, the higher the number, the faster the cloud would appear.
Hope this helps!