How can I retrieve instant messages or text messages accidentally deleted from my iPhone

There are 2 scenarios:

1. If you have iCloud Sync of messages enabled (Settings/[your name]/iCloud - Messages ON you cannot recover deleted messages
2. If you do NOT have iCloud Sync on and you have a computer or iCloud backup of your phone that contains the messages you can restore that backup to the phone. That will overwrite everything on the phone, however, not just the messages.

From a backup made when the messages/texts were present.

Be advised that restoring a backup will over-write everything newer

than the backup. Restoration is all or nothing - you cannot choose

to restore just certain items.

“Instant Messages” are likely associated with some application or vendor. They would govern retrieval.

Unless you house your iMessages and text messages in iCloud, they are saved on your iPhone backup that was created while the messages existed.

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