Vagrant ubuntu/trusty64: ssh or sftp as root?

I need to edit some files over sftp with root rights. I am very confused about:

What is the root user? and how to autheticate it? what is the password, or where are the pub/private keys?

In ubuntu/xenial64 (virtualbox, 20171011.0.0), there's the ubuntu user with sudo access. But doesn't seem able to edit root files over sftp.

I've reading a lot of threads, but still got no clear answer to it.