Edit the file /etc/issue
For example:
# cat /etc/issue
Arch Linux \r (\l)
More options: (preceded by \ )
b Insert the baudrate of the current line.
d Insert the current date.
s Insert the system name, the name of the operating system.
l Insert the name of the current tty line.
m Insert the architecture identifier of the machine, e.g., i686.
n Insert the nodename of the machine, also known as the hostname.
o Insert the domainname of the machine.
r Insert the release number of the kernel, e.g., 2.6.11.12.
t Insert the current time.
u Insert the number of current users logged in.
U Insert the string "1 user" or "<n> users" where <n> is the
number of current users logged in.
v Insert the version of the OS, e.g., the build-date etc.