If you selected email + password authentication, you may need to reset your password for a variety of reasons:
Forgotten
Required by your company's security policy
etc
Reset a forgotten password from the Log In screen:
Click "Reset Password"
Input your email.
Select "Reset Password"
You will receive an email with a link to reset your password.
If you are logged in but want to change your password, log out to go to the Log In Screen.