If there was a substantial mistake in your sent email you should send a correction email: Duplicate the message, correct the error, add a correction notice near the top of the email and in the subject line and send it to the same list.

If there was only a typo or a minor error that doesn't warrant a correction email to be sent then you have two options:

Option one is to do nothing: People who already received your message won't see the corrected version so fixing a typo, etc. won't matter in this case.

Correcting a sent message only matters when a contact clicks the "view online" link in the email (it always loads the most recent version) or if your sent message is included on the archives page.

To fix a typo or other mistake in a sent message, first you need to include the message on the archive page temporarily because only archived messages are editable. You can then make changes and when you're done you can remove the message from the archives page.