Backup and Restore Text Messages from iPhone: http://www.restore-iphone-data.com/backup-and-restore-text-messages-on-iphone-6s-6-plus.html
Backup and Restore iPhone Data without iTunes or iCloud: http://www.restore-iphone-data.com/backup-and-restore-iphone-se-data.html
If you want to backup contacts and other data from iPhone 6s to computer,you can use the iPhone to Computer Transfer tool or iPhone to Mac Transfer tool,which allows you transfer photos, songs, videos, apps, contacts, call histories, notes, bookmarks and more from iPhone 6s/iPhone 6/iPhone 5s to computer. And then you can restore to your iPhone 7/6S/6/5S/5 from backup file.
Read more: How to Switch Text Messages from Android to iPhone 7