Restore Viber Messages from iPhone 6S

No backup = no recovery.

If you backed up to either iCloud or iTunes, you could restore the backup and retrieve the messages.

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

livjenny wrote:

thanks for the reply. I am sure i backed up photos and contacts, but not sure for Viber. is there any way to check it out?

You would have had to perform an iPhone backup to either iTunes or iCloud to have any chance of recovering Viber data. You either did an iPhone backup or you did not. These backups are not selective about what they back up. A photos or contacts backup is irrelevant.

If you did an iCloud backup, you would see it at Settings > iCloud > Backup and then the bottom line displayed would say "Last backup at ..." followed by a date and time.

If you did an iTunes backup, you would see it in iTunes by using the menu to check File (or iTunes on a Mac) > Preferences > Devices

livjenny wrote:

P.S if I chose to restore from backup, the current data on my iPhone will be removed, right?

Yes.