Let's perform the below steps to unpair the AmpliFi HD kit router/mesh points:
 

1. Download the attached firmware file on your Laptop.
2. Plugin the HD Router/mesh points to the power that you want to unpair.
3. Press and hold the reset button for 40 sec (until all of the LEDs are blinking fast).
4. The WiFi with SSID for mesh point will appear as "AFi-P-HD-[end of the serial number]-Recovery" and for the router, it will be "AFi-R-HD-[end of the serial number]-Recovery".
5. Connect a laptop to the Recovery WiFi SSID, open browser (Google Chrome or Firefox), and in the URL enter http://amplifi.lan or 192.168.1.20
6. Then update the custom firmware file for the router and the mesh point that was downloaded in step 1.
7. Once the firmware is uploaded successfully, the devices will unpair automatically.
8. After a successful unpairing process refer to this article to add the unpaired AmpliFi HD router/mesh points to the existing AmpliFi system: https://help.amplifi.com/hc/en-us/articles/360016183893-Adding-AmpliFi-Standalone-Devices-as-Mesh-Points#1

 
I have attached the firmware file for the router and the mesh points.
AFi-R is for the AmpliFi router.
AFi-P-HD is for the mesh points.