Hi frotek,
I would not recommend changing the requirements with Gerome's method... you can just set the values inside the file '/config/application.php'.
'MinUsernameLength' => 4, // Minimum username length.
'MaxUsernameLength' => 23, // Maximum username length.
'MinPasswordLength' => 8, // Minimum password length.
'MaxPasswordLength' => 31, // Maximum password length.
'PasswordMinUpper' => 1, // Number of upper-case letters to require in passwords.
'PasswordMinLower' => 1, // Number of lower-case letters to require in passwords.
'PasswordMinNumber' => 1, // Number of numbers to require in passwords.
'PasswordMinSymbol' => 0, // Number of symbols to require in passwords.
'GMMinPasswordLength' => 8, // Minimum password length for GM accounts.
'GMPasswordMinUpper' => 1, // Number of upper-case letters to require in passwords for GM accounts.
'GMPasswordMinLower' => 1, // Number of lower-case letters to require in passwords for GM accounts.
'GMPasswordMinNumber' => 1, // Number of numbers to require in passwords for GM accounts.
'GMPasswordMinSymbol' => 1, // Number of symbols to require in passwords for GM accounts.
'RandomPasswordLength' => 16, // This is the length of the random password generated by the "Reset Password" feature. (NOTE: Hardcoded minimum value of 8)