I have a client whose site was basically abandoned by the previous developer. I've managed to get ownership of the license transferred and was in the process of updating EE from 2.3.1 to 2.9.0. I followed the instructions to the letter and then logged into the control panel. The only difference was an alert that index.php had changed. Nothing else. How do I activate the update wizard? Is there something missing in the instructions?
Assuming you mean the instructions in the EE documentation then you need to ensure that the installer directory is within the system directory. Also ensure that the system directory hasn't been renamed.
The most important thing Back up your database and files before starting. It is very common for the update process to run into problems when going from a version as old as 2.3.1
You will also find that you may be better off going to 2.8 rather than 2.9. The latest version introduces some changes to the way conditionals work and if your site is complex it might be bit of a job to 'fix' any conditional logic in your templates.