... modify existing page

1. First of all, you can only modify your own pages  and pages for which you have been granted access / rpivilege to edit. Highly privileged users may edit about any page (Drupal records changes in revisions however so, actions are traceable). This being said ...
    
2. OK, Log in first using login box (bottom-left corner of every site page)
3. Navigate to the page you are going to modify.
4. You should see 'View' and 'Edit' tabs right below the page title. Click on 'Edit'.
5. Change the data according to your needs, then click 'Submit'.
6. Changes are applied immediately! Please, check the page view after submit.