are you sure you are wanting to select i think you want to update data. if yes than write update query in place of select query.
like that
SqlCommand cmd = new SqlCommand("update other_source_income set osi_int_from_bank='" + txtOSIInterstFromBank.Text + "' WHERE em_id=" + tb.Text + "", con);
if not than write like this
SqlCommand cmd = new SqlCommand("select osi_int_from_bank from other_source_income WHERE em_id=" + tb.Text + "", con);
and in select command don't use cmd.ExecuteNonQuery()