How to unlock a used iPhone 6 I just bought?

If you can't reach the previous owner whose ID is on the device and have them go to https://www.icloud.com/find/, erase the device, then remove it from their account, then the device is useless. Apple won't unlock it.

Turn off Activation Lock - Apple Support.  *

Are you referring to an activation lock? If so, look at what the previous poster said. Carrier lock? The previous owner will need to contact the carrier and see if they will unlock it (carriers have varying policies). You can try contacting them yourself but they may not help you.