How to change the username of your twitter account..?

1. Login into your twitter account
2. goto -> settings(on top near to search, in the drop down last but first option, just before sign out) -> account(on leftside menu first option)
3. You can see your full account details like username, time zone, mailid etc.. In username textbox type your new desired username and wait for twitter's response regarding the availability of that username.  Try with some modification in the username until you get username available message.  Once you got go to the bottom of the page and save changes. 
Thats all now your username is changed to new one as you entered.


Note: 
1. Details are based on the look of twitter in desktop/laptop web browser not from mobile browser.
2. Once your username is changed you should use that new username while signin into your twitter account.

How to lock a linux user from logging into a machine?

In some scenarios its needed to lock a user to login into his own account.  This can be achieved by many ways.

1.  GUI way

 1.1 Go to System -> Administration -> Users and Groups

 1.2 Now select the particular user from the left side, then click on the advanced settings button

 1.3 Click on the Advanced tab then tick the check box with value "Disable Account"

 1.4 To revoke follow the 1.1 & 1.2 now you can see a "Enable account" button click and enable

2. Terminal way

1. As a root user issue the following command in the terminal

    $sudo passwd -l "username of the user to lock"