To log in on the web app you must fill in your e-mailaddres or username and password. Both the username and the email address must be unique on the system. For personal accounts the username and password can be chosen when creating a user, for an organization both values are automatically generated (username based on first and last name, random password of letters and numbers of 6 characters long).
If you have forgotten your password, and entered an email address, you can reset your password safely by clicking 'Forgot password?' on the login screen and follow the instructions.
Your username and password can also be changed if you are logged in. In the top right of the screen, click on your name and then on profile. On the Profile page, click the Change button and then in the upper right corner 'Change username and / or change password'. If you want to change a field, you have to give the old value first (to prevent unintended changes), and then the new one. When you have entered at least 6 characters in the new username will automatically be checked whether it is still available and the field turns green or red.