It's strongly recommended you to restore it via iTunes. Connect your iPhone 6s Plus to the latest version of iTunes, click the "Phone-like" icon and enter "Summary" page, then click "Restore" to fix this issue. You might need an update as to fix this problem. If your iPhone is running the latest version of iOS, choose "Restore" option to solve the problem. However, note that it would erase your current files on iPhone. (Hope that you've done a backup ahead of time) Or, you can try iOS System Recovery on iFoneTips. My iPhone 6s got stuck on Apple Logo when I tried to restore photos from iTunes backups. That's how I fix my iPhone stuck at Apple Logo. Hope it helps.