Unfortunately once a username has been made it cannot be changed.
If you'd like though you can make a new account and delete the old one.
Since you can only have one account per email address, you can change the email address on the old account (doesn't have to be a real address) and make a new account with the same email address.
Once you've created the new account, on the bottom of the Account > Profile on the old account you can "Delete Account" - keep in mind this cannot be reversed after it has been done.
We suggest doing this after making the new account in case you want to re-follow anyone onto your new account, copying over profile information, and just in case you change your mind before you fully swap over.
Keep in mind though that it is a new account, all followers and subscriptions can't be transferred over