Not sure why your title is "restore". There is no need for a restore. Just run Redsn0w 0.9.6RC17 over your current jailbreak and it will become untethered!
Just for future reference though: To restore your iPhone on 6.15, you will need to create custom firmware with pwnagetool or sn0wbreeze, put your phone in pwned DFU mode using one of these programs or Redsn0w, and then shift + restore ("option" on mac) in iTunes and point to the custom IPSW.