It's not a firmware upgrade; that's a more involved process and is generally never required unless you have a big problem and AP asks you to do that as part of the troubleshooting.
If you have a valid data subscription, then using iFly Connect will work. When you get to the point where you can select the data to download, the top option will always be Weather, and if a software upgrade is avaialble, it will be the second item in the list.
If you don't have a valid data subscription (and it sounds like you don't, if it's prompting you to renew), then you can't get to that screen using iFly Connect. I'm not sure if there is a way to get the latest software without a valid data subscription.