How to change the MAC address of the Ubiquity Edge Router X

Bring up the command line tool (CLI) from the browser GUI. If you use any special characters in your password, you might need to change your password first via the GUI. Despite considerable effort, I did not manage to type special characters into the CLI using Firefox on a Mac (copy-paste does not work). Assuming that you want to change eth0:

configure
set interfaces ethernet eth0 mac 00:03:0D:51:B4:D2
commit
save