I want unlock my iphone 6

You will need to contact the carrier the device is locked to and request the unlock. As an AT&T customer, you can use their online request form to see if you qualify. If you do and they approve it, you will receive an email with directions on what to do. You will end up restoring the device to factory, then you can restore your backup after that, so yes, make sure you have created a fresh backup. https://www.att.com/deviceunlock/#/

Regarding backup, you should have your phone backed up at all times, not just for specific activities. You never know when yor phone is going to be stolen, drowned, run over, suffer a software error from an angry deity... how will you recover your important data from a mangled pile of electronics?