How can I get messages on my MacBook Pro to show 'read' rather than 'unread'?

Delete a message

1. In the Messages app  on your Mac, select a conversation.
2. Select one or more messages (including text messages, photos, videos, files, web links, audio messages, or emoji) in the transcript, press Delete, then click Delete in the dialog.

Note: When you delete a message, it’s moved to the Recently Deleted folder on your device, but not anyone else’s. When you want to delete a message for everyone, use Undo Send.

leomama77 wrote:

I tried this and scrolled down as far as I can - back to 2013 - got rid of about 100 but the rest must date further back and I don't see them in iMessages.

It may have occurred where Migration brought over an extra iMessages file

in Finder >> Hold OPTION key >> Go >> Library

Maneuver to Messages and open it

Look for a file that may have duplicate name with an appended number

If found, to check its' file size use the Command I ( eye )

It would give the size of the file

Example below

Are you referring to text messages or email messages? It's pretty easy with email messages but not so with text messages.