Linux is case-sensitive in usernames.
Historically, in Unix-style systems, the login name (or "user ID" as Debian seems to call it) was up to 8 characters and all-lowercase. In old Unix systems, if your login name included upper-case characters, the system automatically used ALL UPPER CASE on your login session because some ancient terminals did not have lower-case characters available.
Modern Debian can handle longer than 8-character user IDs with mixed case easily, but some programs may still only show the first 8 characters of user ID.
User name, on the other hand, is supposed to be the full human name of the user in question. It is not used for login, but applications may use it if necessary: for example, an email application might format the From: field of emails you write as
From: Jawad Mansoor <jawad@eden>
To change the full name associated with a particular username, use the command
(The command name is shorthand for CHange Finger Name, referring to an old system service called "finger" which is no longer used very often. It could be used to query some basic information on other users. It was a totally unauthenticated and unencrypted service, and so was suitable for secure, trusted networks only.)
For example, if you want to change the name associated with your user id "jawad" back to "Jawad Mansoor", you can do it on the command line by:
Password: <enter your password>
Changing finger information for jawad
Enter the new value, or press ENTER for the default
Full Name [Jawad]: Jawad Mansoor
Room Number :
Work Phone :
Home Phone :
On the other hand, if you want to change the user ID associated to a particular user account, then you'll need the "usermod" command, which is normally usable as root only. I'll use the prefix command "sudo" for getting root access in my examples; however, it is possible to switch to root using the "su -" command or just log in as root on the console, and then use the following commands directly as root without the "sudo" prefix.
Two user accounts cannot have the same name, so if you wanted your current account "jawad" have user ID "neo" instead, you would first have to delete the user account "neo":
sudo userdel -r neo
Then you can change the user ID of the "jawad" account to "neo":
sudo usermod -l neo jawad
If you are logged in as "jawad" while making this change, it might be wise to log out and back in at this point. Otherwise some commands and applications might show errors instead of your user ID. (One old funny error message that might happen in such a situation is "You don't exist. Go away!")
After logging out, you can now log in as "neo": the user ID "jawad" will no longer exist. Note that all the other features of the user account will be the ones the user ID "jawad" had previously: for example, the home directory will still be /home/jawad. If you want to change this, it can be done with
sudo usermod -m -d /home/neo neo
i.e. "move the home directory of user 'neo' to /home/neo"
However, it would be best to use this command while logged in as some other user. (It is possible to yank the home directory out from "under the feet" of a logged-in user, but it is even more likely to cause various kinds of confusion!)
After all these commands I've shown as examples, the default From: line of any emails you might wrote in that system would now look like:
From: Jawad Mansoor <neo@Eden>