Thanks to the TechVideos channel on YouTube

1. Install RoboMongo (now called Robo 3T) from here.
2. Start the UniFi controller (To be able to access the database with the password, the Unifi controller must be running. )
3. Start Robomongo
4. Create a new connection in Robomongo, leave all settings as it is except the port which needs to be changed to “27117″. (Name can be set to UniFi)
5. Click on save so that the settings are saved.
6. Select the new connection in the list and click on “Connect” Now you are connected to the UniFi database.
7. In the left panel select /expand ‘ace’. Then right click on ‘admin’ and select view document.
8. Go here: https://quickhash.com/ (see * note below)
9. Select “SHA-512 / crypt(3) / $6$” as the Algorithm
10. Put the desired password as your Input Data (see below for “password”)
11. Use “9Ter1EZ9$lSt6″ as the Salt.
12. Hit “Generate (Over Secure Connection)
13. Back in RoboMongo, right-click x_shadow and click Edit Document
14. Replace the “x_shadow” value with the value created at quickhash
15. Click Save and you’re done. You can use your new password now

* If you can’t get to the quickhash site, this is what you can use
for the password “password”:

$6$9Ter1EZ9$lSt6/tkoPguHqsDK0mXmUsZ1WE2qCM4m9AQ.x9/eVNJxws.hAxt2Pe8oA9TFB7LPBgzaHBcAfKFoLpRQlpBiX1