What is the purpose of the change? Are you trying to simulate daylighting with clouds? Or solar generation? Then adding a cloud effect can make sense. If you are trying to do actual building energy simulation then you definitely don't want to just change the radiation/clouds because you will not be changing the other atmospheric parameters that also change with cloud cover, namely dry bulb temp, humidity, and wind. If building energy simulation is your end goal, then you want to find actual weather EPW that have the right cloud cover or generate them using a weather simulation.

Thank you so much, Ralph. Well, I have two different external conditions for my research. One included direct and diffuse radiation and another one just diffusive radiation. As you mentioned, if I change the amount of direct and diffuse solar radiation it will affect the temperature as well. By this way, I decided by changing the cloud coverage make these two conditions. For example, when it is clear sky the cloud cover is zero and we have both direct and diffuse radiation, and when the sky is completely cloudy we have just diffusive radiation. This is my strategy. Thanks for your help.