For Beginners - How-to SSH to RPi - Setup Putty in Windows
Once you are on the prompt you can change directory with:
cd /usr/share/dump1090-fa/html
to edit, you can use nano for example, as you need root permissions to edit the file you need to use sudo as well:
sudo nano config.js
(sudo makes you root for the following command only)
Ctrl-x to exit, it will ask you wether to save, say y for yes and press enter when prompted for the path (stays the same).