Salesforce Basics :

Using same List for Update:

List<Account> acList=[select id,name from account limit 2];
system.debug('>>>acList>>>>'+acList);
for(account ac:acList){
    ac.name='Test';
}
system.debug('before update List>>'+acList);
update acList;

Here "acList" contains Updated value



Will It Updates.Parents name field?

List<Contact> contactList=[select id,accountId,account.name from contact limit 2];
for(contact c:contactList){
c.account.name='Update Parent Account';
}

update contactList;

Ans: It will not