To fix the upgrade notice in Safari on older Mac computers that do not support the latest version of Safari please use these steps:
After clicking OK the Yahoo page should reload and you will not be prompted with the upgrade notice during future logins.
- Launch Safari.
- In the menu bar at the top of your screen, click Safari and then select Preferences,
- Click the Advanced tab.
- Select "Show Develop menu in menu bar"
- Go back to the Safari menu bar, click Develop and select User Agent
- Select Other
- Paste in the following user agent string:
After clicking OK the Yahoo page should reload and you will not be prompted with the upgrade notice during future logins.