Double click the SinusBot logo in the tray (next to your clock at the bottom right of the screen) to login. If you need credentials to login from another system, edit the user account or create a new one and set a new password.
Newer releases (0.9.12+) will have a randomly generated password which will be displayed on first run. Those versions can have their password overridden by running the bot via
(make sure to run as the correct system user). Login as admin with the specified password (newpassword in the example above) and change your password to a new one. Afterwards restart the bot without the pwreset parameter.
For older versions there is no option to reset the password; the only options are to either upgrade or to delete the data-folder which will fully reset the bot.