There is no notification system that will tell you when username changes become available. Here is a step-by-step process that will help you understand how to change your username:
- Wait for one of the bi-annual username change threads to be posted. Old threads are not re-used. The threads are posted in the Meta section by Pokemaster. There should be a new one this month.
- There will be a date in the thread posted by Pokemaster. This date signifies the time when the username field in your account settings will become open to change. This date will be a few days after the thread itself is posted.
- You will have until a few days after the username field is first opened to change your username, before it is closed again and you need to wait another six months for the next username change period.
So, you need to wait for Pokemaster's thread to go live and then plan to be on PokeBase at some time during the period that the username field is open, as described in the thread. Usually, this is over a weekend.
Don't check the 'How to change your username' link (and use this instead if somebody asks you how to change their username).