Combine two records into one and update column - linq

Question

I have the following records in my CartItems table:
When I migrate the cart, I'd like to combine the two records into one with the cart id of Test1 and then update the quantity to 2.
So it would be CartItemId 84, CartId Test1, Quantity 2, DateCreated 5/21/2013, ProductId 16
How would I do this in linq/EF?
EDIT
public void MigrateCart(string userName)
{
ShoppingCartId = GetCartId();
var shoppingCart = _context.ShoppingCartItems.Where(c => c.CartId == ShoppingCartId);
foreach (var item in shoppingCart)
{
item.CartId = userName;
}
_context.SaveChanges();
}
EDIT
I think I have it.
public void MigrateCart(string userName)
{
ShoppingCartId = GetCartId();
if (ShoppingCartId != userName)
{
var shoppingCart = _context.ShoppingCartItems.Where(c => c.CartId == ShoppingCartId).ToList();
var userShoppingCartItems = _context.ShoppingCartItems.Where(c => c.CartId == userName).ToList();
foreach (var item in shoppingCart)
{
foreach (var userShoppingCartItem in userShoppingCartItems)
{
if (item.ProductId == userShoppingCartItem.ProductId)
{
item.Quantity += userShoppingCartItem.Quantity;
item.CartId = userName;
_context.ShoppingCartItems.Remove(userShoppingCartItem);
}
}
item.CartId = userName;
}
_context.SaveChanges();
}
else
{
var shoppingCart = _context.ShoppingCartItems.Where(c => c.CartId == ShoppingCartId);
foreach (var item in shoppingCart)
{
item.CartId = userName;
}
_context.SaveChanges();
}
}

Answer 1

as you don't ask how to get the two items, I just give you a really rustic way.
var item1 = context.CartItems.FirstOrDefault(m => m.Id == 84);
var item2 = context.CartItems.FirstOrDefault(m => m.Id == 85);
item1.Quantity += item2.Quantity;//add quantity of item2 to item1
context.CartItems.Remove(item2);//remove item2
context.SaveChanges();