grub2 – can not edit grub file, authorization problem

I used to be in the DOS era, but I confess that LUBUNTU beats me badly.

When installing Lubuntu on my old Sony Vaio, he did not recognize the touchpad nor my wireless logitech mouse. So I installed it using alt commands and the space bar to click.
On many forums, I've found a fix that modifies the grub file in etc / default by adding this:
GRUB_CMDLINE_LINUX = "i8042.nopnp"

I've tried editing FeatherPAD unable to save the file, then the open terminal tried to edit sudo with the error "no authorization", then the GKsudo software, but it seems that it is not installed by default and that I have not been able to install it, terminal ask me for my password you never receive a status message on this entry, is this normal under linux? Could someone help get access to modify it?