Trying to upgrade my iPad to iOS 5.1 but mine doesn't have the software update option in the general menu. What do I do?

From the article you linked, scroll down to the section entitled "Update your device using iTunes". Follow those directions.

You need to connect the iPad to your computer and update via iTunes

iOS 4: Updating your device to iOS 5 or later

The wifi update comes with iOS 5

You don't get the option to update iOS directly from your iPad until you get it to iOS 5. Connect your iPad to your computer and launch iTunes. You'll most likely be prompted to update. For more information and full details, see the section titled "Update your device using iTunes" in this support article:

http://support.apple.com/kb/HT4623

You should read this article as well:

http://support.apple.com/kb/HT4972

Regards.

Forum Tip: Since you're new here, you've probably not discovered the Search feature available on every Communities page, but next time, it might save you time (and everyone else from having to answer the same question multiple times) if you search a couple of ways for a topic, both in the relevant forums and in the Apple Knowledge Base, before you post a question.