Move the pointer over the From field in your message.
Click the pop-up menu that appears, then choose an email address.
If you want to use the same From address for all your messages, choose Mail > Settings, click Composing, click the “Send new messages from” pop-up menu, then choose an email account. Or choose instead to have Mail automatically select the best address, based on the email address of the first recipient in your message, as well as the currently selected Create and use email signatures in Mail on Mac