Read the statement by Michael Teeuw here.
Changing wether API?
Can the weather API be changed? I have an API key for weatherunderground, but dont know which url to use, and do I only have to put it in the config file?
I’m not really sure I understand your question.
What weather module are you trying to use?
https://github.com/cowboysdude/MMM-NOAA also uses the Weather Underground API and it seems to be a very popular module with excellent support from the author