After installing the new shiny 5.7.20 version of the UniFi Controller software you might be faced with an 'Connected (Incompatible Model)' error on your older devices, this is simply due to the EOL being reached.

The device will still function, but no new updates will be installed, settings updated, or statistics reported back.

If you were to attempt to install 5.6 over 5.7 and in a sense downgrade that way, you’ll be faced with 'We do not support upgrading from 5.7.20', to solve this, go ahead with rollback.

Rollback

To perform the rollback you’ll need a 5.6 backup of the controller, as well as SSH access to the box.

First off you’ll need to remove the 5.7 version using the following command sudo apt-get purge unifi -y.

Next up, the repo will have to be pointed back down to 5.6, if not, the install will simply reinstall 5.7 for you, link
sudo echo '#deb http://www.ubnt.com/downloads/unifi/debian stable ubiquiti\ndeb http://www.ubnt.com/downloads/unifi/debian unifi-5.6 ubiquiti' | sudo tee /etc/apt/sources.list.d/100-ubnt.list

Perform an update followed by an install to proceed as per usual.
sudo apt-get update
sudo apt-get install unifi -y

You’ll now have a blank 5.6 version of the controller, access it by https://your.unifi.controller:8443, follow the steps to import the previous backup.

sudo apt-get purge unifi -y
sudo echo '#deb http://www.ubnt.com/downloads/unifi/debian stable ubiquiti\ndeb http://www.ubnt.com/downloads/unifi/debian unifi-5.6 ubiquiti' | sudo tee /etc/apt/sources.list.d/100-ubnt.list
sudo apt-get update
sudo apt-get install unifi -y

You might face an SSL related HSTS error if you’re using Let’s Encrypt on your controller post downgrade, to solve this, set up a new certificate by manually running the cron. /usr/local/bin/renew_lets_encrypt_cert.sh
If all else fails, you should be able to access the controller either by using another browser, incognito mode, or computer to import the backup.