"erase all settings and content" is also called "factory reset". It can erase all the software and setting data at your iPhone, however, your data erased by “factory reset” can be recovered by using professional iPhone data recovery. If there were some important data in your iPhone 6 before, you must permanently wipe them. You must make it through a third party erase software. Here recommend iMyFone Umate Pro.
1. Launch Umate Pro and connect your iPhone6
2. After your device is detected, choose "Erase All Data"
3. You need wait for several minutes for scanning
4. After it finished, input "delete" and click on "Erase Now"