Can I upgrade the rear cog (gears), and will it affect the cadence sensor?

We design the bikes to provide an excellent ride experience so that you do not need to worry about things like this. If you are interested in tuning your bike, there are a couple of options on the cassettes available. Depending on which model you have, there will be a limit to the amount of gears that will fit. You can review this with your LBS, but you must have the 11T as the cadence is measured with the lower gear sprocket.