HIw do I retrieve deleted messages from my iPhone 6

Follow the directions in this link: Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

Still you need to know if your backup was made after you deleted the messages. If you use iCloud backup, this would be easy to determine. You know when you deleted the messages and your phone will tell you when the last backup was made. Go to Settings > Tap your Name Apple ID, iCloud, iTunes & App Store > Tap iCloud and scan down to iCloud Backup. If it is on, tap it and it will tell you the most recent backup. If it is after the messages were deleted, they are gone. If it is prior to when you deleted the messages, you can restore that backup.