You can always add or change something in any material, even after it has been published.

Basically, you edit it in the same way as you do before publishing. To edit a press release, you go to Publish > Press releases, then click on the title of the material you want to edit (or click on the three dots menu and choose Edit).
Edit with your changes. Changes will be updated instantly in your public newsroom – either directly after they have been conducted, of when you confirm via the Update button. That’s it, you don’t need to do anything more.

No distribution emails or alerts will be sent out when updating an already published material, since distribution is only available the “first” time that you publish. In the case that you want to add further distribution to the published content, please see our information about how to send already published content to more recipients.

Emails that have already been sent out with the material, will naturally not be updated, but when a recipient clicks in the email in order to display the material on their browser, they will see the updated web version.

Also, if the material already has been shared on social media, it might require a debug on the social media site in order to refresh the presentation of the material in the social media post.

Please also see:
Publish a press release, news or blog post
In which cases are emails being sent with the content I publish?