If it was a pre-installed app, you can go to Settings > Manage apps, scroll till you find Weather Channel, and then "Uninstall updates." For user apps, about all you can do is check with trustworthy sources (e.g., here, XDA, or a close friend), and if found, uninstall the current (newer) version and then install the older version from the .apk file. Note that the latter method will erase any settings or customizations, so be sure to back them up or write them down first.