Each WhenToWork user can change their own Username and Password. Managers can change their username and password from the SETTINGS page.
The Main Manager also can change any other manager or employee username and password.
Additional managers can change their own manager (and employee) username and password. (If their manager permissions allow them to edit employees, they also can change other employee username and passwords when signed in as manager).
NOTE that resetting the password will expire all previous saved sessions (like bookmarked pages and mobile app icons).