When you logged into Odoo application you will find an option to see who is the current logged in person in the top right corner. This user information have a dropdown button. Click on the dropdown, then you will find a list. In that list select about Odoo.com option. Clicking on that will open a About popup window. In that window, over the top right corner you will find an option like Acivate developer mode. Clicking on that link will reload the webpage.
After reloading it will be in Developer mode. Then the link will change to something like this http://localhost:8069/web?debug=#id=23&view_type=form&model=res.partner