Github has this really cool policy that allows you to ask them to remove dusty user accounts which have not been used for an extended period of time. This means short usernames that have been claimed years ago might be abandoned and ready for a new owner. I was able to get my short username “Nikhil” through Github’s Name Squatting policy which states:
GitHub account name squatting is prohibited. Inactive accounts may be renamed or removed by GitHub staff at their discretion. You can examine if there's activity on an account name and request that support release a name for your use, but keep in mind that not all activity on GitHub is publicly visible. Staff will not remove or rename any active account. GitHub account names are provided on a first-come, first-served basis, and are intended for immediate and active use. Account names may not be inactively held for future use. Attempts to sell, buy, or solicit other forms of payment in exchange for account names is prohibited and may result in permanent account suspension.
Just send an email claiming your desired username is currently abandoned (assuming no private activity) and they will confirm and clear it.
Notice: You will need to update your local repository remotes as well as your domain settings (if you are using github pages hosting).