Click on your avatar, select Settings -> Account.
This is the place where you can change your e-mail and password. Please be aware that the e-mail is your username for login to Productive, so if you change it, you need to use next time you're logging in.
To change a password, put down the current one and then select a new one.
You can also find here an API key and URL to be used by other services to connect to your account.