Hi inmvns,
Thanks for posting your question. I'm more than happy to assist you today. To access that file, you must be a VPS or Dedicated customer with root access. If you do not have root access, you can request it through AMP. Here's a link explaining how to gain root access to your server:
Root Access
Also, you will need to use SSH to edit the file. Here's a good tutorial on using SSH:
Using SSH to access my server
Once you have set up SSH access to your server and have root, you can use a program called Nano to edit the file via SSH. Here's our tutorial on using Nano:
Using Nano to edit files via SSH
I hope this helps! if you need further assistance please feel free to contact us.
Thanks!
Tim S