Unfortunately you cant! It is totally impossible to change one's username here on Steem.
It is worth to note that Blockchains are "immutable". This means that data or information posted to the blockchain will be their forever and somewhat locked for eternity. The "Edit" button you see on your posts and comments are just edited out visually through the platforms UI or front-end but not edited out on the block level. If you look at any Steem block explorers, you will see that it will just create another block for that edit.
By that logic, if one is to change his/her username on Steem that would mean editing out hundreds or even thousands of blocks just to change one data since you have to change all your previous blog posts and comments to your name too. This is just not efficient.
No, you can't.
Your username is the unique identifier of your Steem account. You can create another account under a different name if you want.
And Steemit.com is nothing but a user interface that allows users to post transactions to the Steem blockchain and request data from the blockchain and show it to the user in their browser. All the accounts, posts, comments, money transfers and all that is on the Steem blockchain. Steemit.com only allows you to access it.
No. But you can create another account and power your old account down to your new one.
I don't think this can happen. It's the watch the steem blockchain was configured. I'm wondering though why you'd want to change the username.
If you could, I'd say you get another account and make a post saying you've moved to a other account so your followers can be alerted.
Best of lucks mate!
Definitely not. What you can do though, is to make a new account with the desired name and transfer your SP there.