Not sure if you are asking for updating passport info or just CSE in general.
Regarding the passport, I have read people have raised a CSE and when they get the PPR, they send both passports just in case, the expired one so they can check the info in your application and the new one so they can stamp your visa on it.
Regarding CSE in general, you get to write 1500 words I believe, and then you have the option to attach some documents, they will ask for name, email, DOB, UCI and application number, after you send the CSE you will receive an automated response saying you filed a case through the webform and a couple weeks later (mine took 3 weeks one, 25 days or so the other, even got it on a weekend) they will actually answer it, usually a generic message saying they received the info and X and Y documents, that they have forwarded them to the responsible department and that they will contact you if they require further information, however nothing will change on your gckey profile, at least mine has not so far, not even the additional documents line.