Question

 var main = (from d in db.Discounts where d.Id == discount.Id && d.UserId == userId select d).FirstOrDefault();
main.Address = "new address";
main.TermsStocks.Clear(); // I need clear collection and add new object.

            foreach (var termsStock in terms)
            {
                main.TermsStocks.Add(new TermsStock() { Id = Guid.NewGuid(), MainId = main.Id, Title = termsStock.Title });
            }
db.SaveChanges(); 

and I have error:

The operation failed. Unable to change the link because one or more
properties of the foreign key values ​​do not allow NULL. If you
change the connection property of the respective foreign key set to
NULL. If the foreign key value does not support NULL, must be defined
a new relationship, the foreign key property should be set to another
value other than NULL, or to remove unbound object.

how to remove the entire collection and save a new one?

Answer 1

You will have to remove those items form the database as well, not just from the nav property.

Roughly:

// main.TermsStocks.Clear(); 
foreach (var item in main.TermsStocks)
{
    db.TermsStocks.Remove(item);
}