1. Yes, you can.
2. Yes, a valid/eligible profile is editable until it is:
i) rejected by the system
ii) withdrawn by you
iii) no longer valid (expired - over a year old)
iv) has been granted ITA
3. No need to withdraw until unless you are considering changing the basic details. An Express Entry profile can always be edited; with true information of course.(including changes in education information, IELTS/CELPIP or language profiency score etc.)