var query = from s in students.Students.Include("Groups")
                            where (s.C_Lastname == strLastName)
                            && (s.C_Name == strFirstName)
                            && (s.C_Middlename == strMiddleName)
                            && (s.Groups.Faculties.С_Name == strFac)
                            && (s.Groups.C_Code == strGroup)
                            select s;
                foreach (var s in query)
                { 
                    s.C_Lastname  = textBox2.Text;
                    s.C_Name = textBox3.Text;
                    s.C_Middlename = textBox4.Text;

                }

int intKey = 1;
s.GroupsReference.EntityKey = new EntityKey("StudentsEntities.Groups", "GroupID", intKey);