When starting the computer, hold down the SHIFT key and wait for the GRUB prompt.
I needed this as I had to edit a parameter that seemed to have been added in a recent upgrade.
press the E key and you are taken into the edit process for the config file.
Find the parts that you want to change, maybe just deleting an option, and then press F10 to continue the boot process.
The config file is not changed at that point - you have to do that separately - more later.
To make the change permanent we have to use something equivalent to:
