This is a more detailed guide but may not have enough info. Are you familiar with using ssh to access the router, either using command prompt or PuTTY (or terminal on a Mac)? That’s the first step, then you’ll need to use a text editor (vi is my editor of choice, but the interface is… Not good)? If you can get into it via ssh, I can write a one line sed command for you that will make the appropriate changes.