How do I regenerate the text version of an email after editing the HTML draft?
Welcome to Stack Exchange, CiviMail will create a plain text version of your email for those using a plain text client. So there is no need to enter content in the plain test area unless you wish folks using plain text to have a specially formatted message or you just want to send your email in plain text that is when you would use the plain text field.

If you have already entered text, delete it and it will be re-generated.

Here is a link to more info: https://docs.civicrm.org/user/en/latest/email/mass-mailings-using-civimail/#mailing-tab

Hope this helps