UPDATE statements need data to actually put into the fields, and it is a very good idea to limit the fields to just those you want to change:
using (SqlConnection con = new SqlConnection(strConnect))
{
con.Open();
using (SqlCommand com = new SqlCommand("UPDATE myTable SET myColumn1=@C1, myColumn2=@C2 WHERE Id=@ID", con))
{
com.Parameters.AddWithValue("@ID", id);
com.Parameters.AddWithValue("@C1", myValueForColumn1);
com.Parameters.AddWithValue("@C2", myValueForColumn2);
com.ExecuteNonQuery();
}
}