How do I restore a text message conversation after accidentally deleting it?

If the conversation was include in an iTunes or iCloud backup, then restore from that backup:

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

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

Otherwise the deleted message conversation is lost permanently.