The 8 ohm tap is a wire that terminated on the board (I believe its green if my memory serves me correctly) over near the existing output jack.
You could hook up a heavy duty SPDT switch to switch between the 2 taps (the 16 ohm obviously goes straight to the jack now, but would be moved to the switch). But...i'd recommend just adding an extra output jack (or maybe 2, to make the amp more versatile). You can run the 8 ohm tap to the extra jack(s).
The reason behind that (as opposed to a switch) is that you'd be able to run the internal speakers WITH an external 16 ohm load should you ever want to, and that's the least instrusive way to do it. I did this to my Korg RI a few years back and it's very convienant to be able to run an extension cab with the amp.