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!
       
               
                     
                              crwdns2934105:0crwdne2934105:0
                     
      
                      crwdns2934113:0crwdne2934113:0
                   crwdns2915270:0crwdne2915270:0
      
   
   
      crwdns2889612:0crwdne2889612:0
            4
   
 
            
               
                   
               
               
      
         
crwdns2947414:01crwdne2947414:0
If you do get them all apart, I would encourage you to document the process. http://www.ifixit.com/Guide/new
crwdns2934271:0crwdnd2934271:0 Alex Jansen crwdne2934271:0