How to retrieve deleted messages

If you are using Messages in iCloud, then I am afraid the messages are going for good. If not, then you need to restore your iPhone from a backup that has those messages:

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

Be aware that restore is an all or nothing process. You cannot restore only messages (for example) and any information on your iPhone between now and when the backup was created will be lost.