Just a note to explain some of the shorthand here.
Firstly, in Linux you normally operate as a limited user. So on my machine I work as 'bbfuller' not surprisingly. That does not allow me to do lots of the administrative things on my computer. There is a user 'root' sometime referred to as the superuser who is allowed to do all tasks on the computer and who's rights we sometimes need.
One of the ways to obtain those rights is to issue the command:
that's a space and a minus sign after the su.
When you issue that command you should be prompted for the root password that you were asked to provide at install time.
When you do you will see that the prompt on the command line changes. On this machine it is:
before the command,
In the command cd ~<yourusername>/Desktop
The cd command is one for changing directories. Usually you specify the path to the directory you want to change into either completely from the / directory, or to a directory that is either directly above or below the one you are in.
The shorthand '~' in the command means move into the home directories. That is it saves you having to type /home and a username
If you issue the command in the form it is given above though when you have acquired root privileges with su, it will look for the folder and file you specify under the user root's home area, whereas the file you have stored is probably on your own desktop.
So the correct form for the command to find the file on my machine would be:
You will of course have to substitute your username for mine in the above command.
Incidentally, if you see the shorthand <yourusername> in a command it means substitute your username here.
So although in writing on paper or on the forum:
are the same thing, when issued on a machine the former would work, the latter would fail.