To help address these concerns, we've done a one-time refresh of the free BattleTag name change feature on all Battle.net accounts, so that everyone once again has one free BattleTag name change to use at their discretion.
This means that if you already used your original free change, you will now have another opportunity to change your BattleTag nickname. If you did not use your free BattleTag name change or have not yet created a BattleTag, you will still only be able to change your BattleTag once for free.
To set or change your BattleTag nickname, log in to Battle.net Account Management, head to the Account > Summary page, and look for the Create or Change option under Account Details. As a reminder, BattleTags aren't unique, so you can pick any name you like as long as it adheres to the BattleTag Naming Policy. For more information, see the BattleTag FAQ and Battle.net website page.
Remember, your BattleTag serves as your public identity when playing Diablo III, meaning you'll be identified in groups and in chat by your BattleTag -- choose your BattleTag wisely!