Before join a team you have to select your profile name.
If you have already changed your name once, there is no way to change this again.
There is a warning of this before you click confirm.

If you have not changed it before, do the following:
1. Click on the Options button in the top right hand side of the screen.
2. You can see a button next to your current name saying “CHANGE”.
3. Once you have selected this new name, you will not have another chance to change it, so make sure you choose a great name the first time!
If the option is not here, then you cannot change your name.