How can you retrieve accidentally deleted messages?

Go to Settings/[your name]/icloud. Is the switch for messages ON? If it is there is no way to recover deleted messages.

If it is OFF and you have a backup that was made before the messages were deleted you can restore that backup. Doing so will overwrite everything on your phone, not just those specific messages.

Assuming you don’t house your messages in iCloud, they are saved on your iPhone backup. Restore that backup to retrieve the messages that were present when the backup was created.

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support