There are some pitfalls when retrieving CRM entities with LinqtoCRM and trying to update them through the CRM web service. The most intuitive (but wrong) approach would be this:

var res = from c in p.Linq()
		  select c;

foreach (var con in res)
{
	con.address1_line1 = "foo";
	service.Update(con);
}

This fails unfortunately. I think someone at Netcompany (my former employer) worked out why this was at some point, but I’ve forgotten.

Instead what you want to do it is new up new entities yourself while setting the relevant id attribute, and then updating the attributes you want to change:

var res = from c in p.Linq()
		  select new contact() { contactid = c.contactid };

foreach (var con in res)
{
	con.address1_line1 = "foo";
	service.Update(con);
}