global class updatePhone implements Database.Batchable<sObject>
{
    global Database.QueryLocator start(Database.BatchableContext BC)
    {
        String query = 'SELECT Id,Phone FROM Account';
       
        return Database.getQueryLocator(query);
        
    }
     global void execute(Database.BatchableContext BC, List<Account> scope)
    {
        for(Account a : scope)
        {
            a.Phone = '999999999';
        }
        Database.update(scope);
    }
    global void finish(Database.BatchableContext BC) {
    }
    
}