Its just like returning to factory defaults on a router, modem, NAS etc. Look at the bottom rear or the phantom (the undercarriage where the VPS and uktrasound is. There will be a red light on the side with a small hole next to it where you can poke the end if a ball point pen, paperclip (un bent), toothpick, the SIM tray openening tool that came with your IPHONE etc into it. Push it in (you dont have to push hard) for three seconds or so and release then enter the link aircraft setting in your GO App (remote needs to be on and connected to App).
Im sure its explained clearer in the manual however this is all there us to it.
Fresh AC do an IMU followed by gimbal calibration (even if it says not needed) and get a good compass calibration in before you fly.