You can't.
Activation Lock is a security measure to prevent unauthorized use in the event that the device is lost or stolen. If you were able to bypass it somehow, it would defeat the purpose of the feature.
To remove Activation Lock, the original owner (the person who enabled Find My iPhone) needs to remove the device from their account, either directly on the device or from iCloud.com. If you can't speak to the original owner and ask them to do this, the phone is an expensive paperweight.
I suggest you only buy an iPhone from an authorized reseller, if not Apple directly.