Your Chromebook must be in developer mode before you attempt any of this.
To put your Chromebook in developer mode, follow the instructions under the 'Entering' section on this page. Note that the instructions work for most hardware types, not just the Acer C720. That said, it's worth a quick google search to see if there are any instructions specifically for your model of Chromebook.
You'll need to complete the instructions at this page under the heading 'Mounting Root Filesystem as Read-Write.
This guide assumes you already have a chroot that you can enter to follow the instructions. If you don't, visit the crouton page to get one going.