airOS 5.5+ and airOS 7 come by default with HTTPS enabled (instead of HTTP), because of that you will see a security alert every time you access the unit's Web UI. That doesn't compromise device's security, but it could be a little bit uncomfortable for some users. If you want to remove this security alert you will need to replace the existing certificate with a trusted certificate, you can get one from any Web hosting provider, most of them offer SSL certificates.
Steps to Replace the Certificate
1) Upload your trusted certificate/key files (server.crt and server.key) to the /etc/persistent/https directory in your airOS device. Make sure server.crt and server.key are for the correct device host name and IP (http://en.wikipedia.org/wiki/
Example: Assuming you have server.crt and server.key file on your local machine in the /etc/https directory:
scp -r /tmp/https ubnt@<device ip>:/etc/persistent/https
2) To save these changes to persistent memory:
ssh to device and execute
save; reboot -f
After reboot web server will use new signed cert.
Note: if files are invalid (bad format or something) there will be no access to the web server.
To clean up them do from cmd: rm -fr /etc/persistent/https; save; reboot -f