The only way is to sign out of one and then sign into the other. Here’s how:

1. Click Sign Out just below "Welcome, yourname" at the top of the page.
2. This returns you to Yahoo!’s home page. Here, click Mail to sign into the other account.

Using more than one account: You can use the same computer for multiple Yahoo! Mail accounts, but it isn't possible to be signed in to more than one account at a time in the same browser.

If you have two different browsers on your computer—for example, Firefox and Internet Explorer—you can open both browsers to sign in to two different accounts.