Gain access to the box running the service, same as how you get to issue the commands to start and stop and get status for the service.
This is usually by either
- sitting at box with monitor and keyboard
- remote access over the network
If you are using a keyboard and monitor at the computer running the service, start a terminal and edit the file with an editor, for instance
$ sudo nano /path/to/file
nano is a simple editor with prompts.
Make the changes and save the file
You can use any editor you like.
If you are remote from the computer running the service, use ssh (putty on windows) to get terminal access and proceed as before. You can also use a remote desktop instead of ssh if you have that set up.