keychain-update.swift

// Filtro para encontrar el registro
let query: [String: Any] = [
	kSecClass as String: kSecClassGenericPassword,
	kSecAttrService as String : self.serviceName,
	kSecAttrAccount as String: user.name
]

// La clave que queremos actualizar
let fields: [String: Any] = [
	kSecAttrAccount as String: user.name,
	kSecValueData as String: passwordData
]

// Ejecutamos la operación
let status = SecItemUpdate(query as CFDictionary, fields as CFDictionary)

// Comprobamos el resultado
if status != errSecSuccess
{
	throw KeychainError.unknown(status: status)
}