Recently, Twitter has undergone some notable changes in its web interface, that removes the username from the reply. This makes the process more complicated sometimes.
For example, consider a situation where I want to reply my friend's tweet, but he also mentioned another user. However, from now, you don't need to mention that other user, which is included there by default. Previously, you can delete the username just by selecting it and hit the delete button.
From now, if you want to delete the username, you start by clicking the reply button below the tweet that you want to reply to. While replying you will notice the usernames in the “Replying to” box.
If you only want to reply to any one of the users who tweeted and leave out other, click on the usernames next to “Replying to”. To remove them from your reply, click on the checkbox next to the user to uncheck it.
Now you can close the dialog box by hitting the close button on top right. You're done!