How to access files without being logged into the operating system

[PowerGraphics]

I have an Indy R4400 32 MB RAM 1GB HD 8bit, IRIX 6.2.
I changed the file /var/X11/xdm/Xservers, experimentally with the -class PseudoColor parameter, removed the -c option, and did not add the -depth parameter.
Now after booting, everything on the login screen is black.
I made a copy of the Xservers file to Xservers.back previously.

What is the easiest method to edit and/or copy the original file back, without being inside the operating system?

Is it possible to access via PROM?

 

[Elf]

You could mount the disk partition from a shell on the install CD, but the PROM doesn't really understand the filesystems to any significant degree though.