To update records, you'll need to issue an update statement against sObject(s) that already have a value for the Id field.

Bouncing off the last code example. Say we're asked to find the most newest Account and update the description to say ''Latest Account":

// Retrieve the Account record from the database 
Account latestAccount = [Select Id, Name, Description FROM Account ORDER BY CreatedDate DESC LIMIT 1]; 

//Set the description field 
latestAccount.Description = 'Latest Account';

// issue the DML operation 
update latestAccount; 

3 lines! That's all it takes. This worked because the Id field is part of the latestAccount variable set when the SOQL runs. This would not have worked:

// Creating a sObject in memory 
Account latestAccount = new Account(Name='Apple Inc');
latestAccount.Description = 'Latest Account';

// issue the DML operation 
update latestAccount; 

We'll get back a DML Exception. System.DmlException: Update failed. First exception on row 0; first error: MISSING_ARGUMENT, Id not specified in an update call: [].

The Id must be specified. The same is true for lists of sObject records:

List<Account> accountsWithoutType = [SELECT Id, Name, Type FROM Account WHERE Type = ''];

for (Account acc : accountsWithoutType) {
    acc.Description = 'Basic';
}

Account newAcc = new Account(Name = 'Apple', Type='Basic');
accountsWithoutType.add(newAcc);

update accountsWithoutType;

Now that doesn't mean you can only update sObject records that are set from a SOQL query.

public static void updateAccountDescriptionById(String newDescription, Id existingAccountId){
    
     Account acc = new Account();
     acc.Id = existingAccountId; 
     acc.Description = newDescription
     
     update acc; 
}

This works! The id field just needs to be set with a valid Id. It doesn't matter how it gets set.

You'd still get the same System.DMLException. All the records being processed by the update statement must have an id.