I am new to thermal modelling but maybe I can help you.
I don't understand if your problem is how to change the thermal parameters (in this case just double click on the thermal chain) or how to chose them.
It depends a lot on your topology and hardware. Typically, in my models, I create the thermal model through the component datasheet and then add it to the thermal description. (Everything is written in the Plecs manual)
For example, in IGBT component, I added the Foster or Cauer datasheet values so my heat sink block will give me the case temperature. Then I have to model the real heat sink, the thermal grease and the water cooling panel. Every component has its thermal capacitance and a single thermal resistance could be not exact.