You are simply going to have to ask the client for this information. Some clients are going to be leery of providing this access. In this case, you are going to need to have a clear and honest conversation with the client. Remind them how they have trusted you with this search engine optimization campaign. Certainly, they can trust you with access to their content management administration system, too. If they refuse, then you will need to work with their web developer, if they have one. Implement any required changes for the client’s site to meet current search engine optimization specifications.