As you know, you can restore your iPhone 6s with your backup file to get back the deleted messages. If you don't have backup file, you can only scan your iPhone and find the deleted messages with iPhone Data Recovery. This software can help you to find and recover deleted text messages on iPhone. You can learn how to retrieve deleted text messages on iPhone
here. Besides messages, you can also recover deleted photos, videos, contacts, call history, notes, reminders, calendars, voice memos, voicemail, safari bookmarks and other deleted data from iPhone, iPad and iPod touch.