Go to the Yahoo! mail web page. Once there, copy the URL or web address from your browser's address bar. You can do this by highlighting the address and then right-clicking with your mouse. Choose "Copy." When you're done, look for "Internet Options" if using Internet Explorer or "Options" if using Mozilla Firefox. Under the "General" tab, the usual first option is to create a homepage tab. Just press "Use current." This will replace the old homepage to the Yahoo! Mail page.