On every calc sheet there is a login button on the top right of page.

Click on the login button and it will direct you to the login page.

You can also click on this link  to go to Login page directly.

On login page, the Keep me signed in is checked by default.
With this option being checked you will automatically sign in
every time you start the browser.

If you work on a public or shared computer, you may have
the Keep me signed in uncheked  to avoid other people
stealing you account.

How to Change User Name and Password

After login, scroll to the bottom of login page and you can change your user name and password from there.