I tried sudo firmware upgrade but it says “unknown commando”
sudo upgrade
sudo get firmware upgrade
try “man dfutool”, this gives you the man[ual] page for dfutool, which probably does what you want 
p.s: if you ever get stuck type “apropos <keyword>”, this searches names of programs and short descriptions.
also, the commands NEVER have spaces in them, and the names are usually short, like DFUtool being device firmware upgrade tool.
just type “cd /” and then “ls”…
The way the question of the OP is worded, I doubt he knows his way around linux, nor the proper terminology and he maybe doesn’t want to do anything with firmware, but rather update his Ubuntu release from Version XY to, well most likely the upcoming 14.04 LTS?
Sounds like he meant to ask:
“What is the terminal command to update my Ubuntu release to the latest version?”
?
I know only limited Linux commands, but the way I do it is ‘sudo apt-get update.’ This probably only works on Raspbian Wheezy.
‘apt-get’ and ‘aptitiude’ only work with Debian derivates as this is part of Debians package management.
Also “sudo apt-get update” only updates your repositories source list.
In order to install them you need to use ‘sudo apt-get install’.
And as Debian is no rolling distribution, nor are 'buntus, you can only re-install a new version.
You could also update the repository to the latest version and try a ‘sudo apt-get dist-upgrade’ which usually ends in an unstable, unresolved-dependency chaos.
That’s why most of the OOTB Debian derivates offer to “backup your data”, then you install the latest LTS version, for Ubuntu the 14.04 LTS released today, and then re-import your “backup”.
That’s the cleanest and safest way unless you use rolling/source based distributions.
Alright I’m back home. Gotta upgrade the OS now. I’ll try those commands.
arch is rolling-release, but hans probably doesnt want to deal with it.
apt-get will only update/install/clean/remove packages, not actually change the kernel and so on.
so if you just want your packages to be up-to-date, sudo apt-get install <package> or sudo apt-get update should work.
if you want a new OS, i (basically) unpack the iso of the distribution (you can use firefox or chrome or whatever for getting the iso, most dist’s have a function to unpack iso’s i think), put it on a USB, boot off the usb, and on mint for example there is an “install” option somewhere on the desktop i think.
alternatively you can rewrite the kernel 
There are plenty OOTB Linux with rolling-releases for desktop users:
Sabayon Linux - Gentoo based OTTB rolling release distro
Tanglu Linux - Debian! based OTTB rolling release distro
…
Wrong - sure it will:
~>apt-cache search linux-image
Besides that you can always configure and compile your own kernel.
Just for the sake of correctness: You always do it the other way round. You want your package index updated and then install the latest version.
“Update” still confuses users:
“update” does not update any package, it updates the index list of packages.
“install” installs a selected package.
“upgrade” updates all packages where the new index you created with “update” holds a newer package version than the one you have installed.
“dist-upgrade” does “upgrade” but resolves dependencies a lot more brutal. It drops dependencies of “lesser” packages in favor of system critical packages, thus usually leaving a mess. The dependency resolver of “aptitude” is a tad better with the “full-upgrade” version.
But like I said, if you change the repository of a Debian based distro to another version and try to upgrade you either know exactly what you’re doing and even then you usually end with a complete mess.