It's not too difficult once you know that Facebook stashes all your events -- whether you've RSVP'd that you're going to attend or not -- on your Facebook Events page.

Which leads to the obvious question "okay, where the heck is THAT page?"

As with much of Facebook, it's more obvious than it seems, but due to the ever-increasing complexity of the user interface, we're learning to ignore the navigational elements sitting in front of us.

This is a case in point...

Look on the left of your regular Facebook page:

As is highlighted in the screen capture, click on "Events" and you'll see what's going on today, tomorrow, and further into the future:

I'm going to scroll down a bit and find a Peace Corps' related event to which I was invited:

Here you can see that my RSVP was "maybe". To change it, click on the current RSVP status -- in this case "Maybe Attending" -- and a small window pops up with details of your response:

Ah, now you can see how to proceed, right? Click on the appropriate changed RSVP option, perhaps add a comment to explain what's going on.

... and click on the blue "RSVP" button to update your event status.

Scroll down a bit on your Facebook Events page, and now you'll see the status for this particular event has updated appropriately:

That's all there is to it. Not too difficult.