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where is the location of the WIFI flex cable and is there a tutorial on how to replace it? thanks

There are 2 WiFi antennas: iPhone 6 Plus Wi-Fi/Bluetooth AntennaiPhone 6 Plus 5GHz Wi-Fi Antenna If you need to replace the 5Ghz antenna, then use this guide and if you need to replace the regular WiFi antenna, use the Rear Housing guide to see how it is installed (there are no specific steps but look at Steps 65-66 to see where it is)