At the top are some icons, slide them over till you see the cog wheel:
at the bottom, you will see 2 buttons, one which binds your account so you can also access the game in your browser, and the rename feature:
I strongly suggest that before logging into the game via your browser that you go directly into your HCS account via the forum using the same log-in information used to sign up on the app, and link your account to all other available log-in methods. This should minimize the chances of having a second account made the first time you use the browser version of the game.
- Mzzery, Thoran and Grumpy Grey like this