Hostname and domain in ubuntu linux are displayed follwoing the pattern:

username@hostname.domain

In order to change the latter two open a terminal and type:

sudo hostname host.domain

replacing host.domain with your own e.g. laptop.aldeby

Ensure that the changes are also reflected in the following files:

/etc/hosts
/etc/hostname
/etc/resolv.conf

The latter is only needed if you have edited it before, otherwise simply reboot your computer or restart networking services.