For the V3 version of the template set, it's really easy to change weather software:
1) download the plugin (for Weather-Display, the WD-plugin.zip)
2) follow the customization info for both Weather-Display and the Settings-weather.php (the Settings-weather.php from the WD-plugin replaces the Settings-weather.php from the CU-plugin for Cumulus)
and you're done. Note that some new pages now work with WD enabled while some older Cumulus-only pages no longer work.. this is normal
If you rename your Cumulus Settings-weather.php to Settings-weather-CU.php before replacing it, then you can still view a page using Cumulus by appending ?wx=cu to the page in the browser. I use this little trick for testing multiple weather software on a single site
Best regards,
Ken