How to stop nginx from using port 80

You need to explicitly define a listen parameter other than 80 for every server block in /etc/nginx/nginx.conf, otherwise nginx will silently use port 80 by default.

Also, adjust the configuration in /etc/nginx/conf.d/default.conf.

Open /etc/nginx/sites-available/default

nano /etc/nginx/sites-available/default

Change port 80 in there to something else and save your changes