Backup iphone on Itunes without overwriting the previous backup
How can I do a backup of my iPhone without overwriting my previous backkup
iPhone 6s
If you have a Mac go to iTunes Preferences, Devices tab, right click on the current backup and archive it.
For Windows it’s much more complicated; you need to find the backup and make a copy of it. See→Locate backups of your iPhone, iPad, and iPod touch - Apple Support