how to get a guid of a record to update

my code is // update a existing Account  record which update a record linked with contact record also //

if (entityAccount["parentcustomerid"] != null)
{
Guid parentId = ((EntityReference)entityAccount["parentcustomerid"]).Id;

QueryExpression query = new QueryExpression();
query.ColumnSet.AllColumns = true;
EntityCollection entityCollectionContact = _service.RetrieveMultiple(query);
foreach (Entity entityContact in entityCollectionContact.Entities)
{
if (entityContact.LogicalName == "contact")
{
if (entityContact["address1_city"] != null)
{
entityAccount["address1_city"] = entityContact["address1_city"].ToString();
}
if (entityContact["address1_line1"] != null)
{
entityAccount["address1_line1"] = entityContact["address1_line1"].ToString();
}
if (entityContact["address1_line2"] != null)
{
entityAccount["address1_line2"] = entityContact["address1_line2"].ToString();
}
if (entityContact["address1_line3"] != null)
{
entityAccount["address1_line3"] = entityContact["address1_line3"].ToString();
}
if (entityContact["address1_country"] != null)
{
entityAccount["address1_country"] = entityContact["address1_country"].ToString();
}
if (entityContact["address1_stateorprovince"] != null)
{
entityAccount["address1_stateorprovince"] = entityContact["address1_stateorprovince"].ToString();
}
_service.Update(entityContact);
}
}
}