global class UpdateContactAcoount implements Database.Batchable<sObject> {

     global Database.QueryLocator start(Database.BatchableContext bc) {
        return Database.getQueryLocator(
            'SELECT ID,name,Accountnumber, (SELECT ID, lastname, accountid FROM Contacts) FROM Account Where Accountnumber = '123''
        );
    }
    global void execute(Database.BatchableContext bc, List<Account> scope){
    account a = [select id,name,(select id,firstname,lastname from contacts) from account where name=:'Test'];
            List<Contact> contacts = new List<Contact>();
        for (Account account : scope) {
            for(contact c : account.contacts){
            system.debug('Contact:'+c);
            c.accountid = a.id;
            contact eCopy = c.clone(false, false, false, false);
            contacts.add(eCopy);
            }
        }
     insert contacts;
    }
    global void finish(Database.BatchableContext bc){
    }
}