Too many double quotes!
Change:
oleDbDataAdapter1.InsertCommand.CommandText =
"insert into magazina(sasia) values ('" + sasia_totale + "') where emri = '" + listBox1.SelectedItem + '"";
To:
oleDbDataAdapter1.InsertCommand.CommandText =
"insert into magazina(sasia) values ('" + sasia_totale + "') where emri = '" + listBox1.SelectedItem + "'";