Which is the proper port specification in docker-compose?

[Royal2000H]

Hi, I don't have a lot of docker experience and even less docker-compose so please bear with me. I will be setting this up on a Synology NAS which already uses port 5000 so I'll need to change the port.

The docker-compose.yml has both:

  #    environment:
  #        Default listening port, can also be changed with the -p option
  #      - PORT=5000

And

      # Comment out ports: when using behind a reverse proxy , enable networks: etc.
      ports:
        - 5000:5000
      restart: unless-stopped

Say I want to use, for example, port 1234. Am I supposed to comment out the first block and write PORT=1234?
Or modify the second block, in which case, is it 1234:5000?
Or modify both? I just don't understand what each part is responsible for.

Thank you

[Royal2000H]

OK this was easy, just change both:

      environment:
  #        Default listening port, can also be changed with the -p option
        - PORT=1234

      # Comment out ports: when using behind a reverse proxy , enable networks: etc.
      ports:
        - 1234:1234
      restart: unless-stopped