I recently had to work with a Sitecore instance where one of my colleagues – who has gone – had not documented the password.

The quickest answer I could find to this was to reset the password to its default:

-- Might want to check that 'B09BBCDE-9B3C-4DCD-B69E-67F615CE01E9' is your Administrator
Update [aspnet_Membership]
set [Password] = 'qOvF8m8F2IcWMvfOBjJYHmfLABc=',
[PasswordSalt] = 'OM5gu45RQuJ76itRvkSPFw=='
where UserId = 'B09BBCDE-9B3C-4DCD-B69E-67F615CE01E9'