First, open the Mail app on your Mac if you have not already done so. Now click on "Mail" in the menu bar at the top of the screen and select the entry "Settings" from the list.
In the settings you will now see different tabs. However, select the "Compose" tab. In the tab you will now see the point "Send new e-mails from". Click the drop-down menu behind it and then select the desired email address.
You can now leave the settings again and from now on, every new e-mail will be sent by default from the selected account.
Note : If you write a new e-mail, you can of course still choose another e-mail for delivery. To do this, click on your e-mail address in the new mail and then select a different address from the drop-down menu.