Your main password signs you in to the Vault and can authenticate compatible Files and Calendar clients. It does not authenticate external email or contacts clients.
Use a long, unique password that is not used for any other site. A password manager can create and store it. Do not send it by email or enter it on a site other than an expected LightDrive sign-in page.
Change the password under Settings → Security → Password & sign-in. A Files or Calendar client that stores the main password must be updated afterward. Vault app passwords, Email client passwords, and Contacts-only client passwords remain separate and do not change with the main password.
If you forget the password, use the recovery link on the Vault sign-in page. Instructions go to the verified recovery email address.