I don't have a service manual for you, however I've taken apart my Powershot S500 several times, including opening the lens/sensor assembly to successfully clean dust on the sensor.
I documented it in this teardown report here on ifixit. From other Powershot cameras I have seen and poked around with, the construction of others is very similar so even if you're wanting to do this with a different model, you may find my photos instructive.
I also notice there are some additional repair guides for other Canon models here on ifixit now - take a look at those also.
if you are handy with taking apart things and handling tiny screws and delicate ribbon cables, you may be able to take apart your camera without a service manual. basically just take it apart one screw at a time, save and label where the screws go, and perhaps take photos along the way (that's how my teardown photos came about). then reverse the process when done. good luck!