C# :: Update Loop In Unbounded Datagridview

I have a data grid view

ProductID|Name|Category|Description|quantity
1|Coat|men|black coat|5
2|Jacket|men|green jaacket|2
3|Pillow|Home|white|10

a user can edit quantity ie productID 1 has quantity 10, and ProductID 2 has quantity 3, it should update once clicked update button. however i must highlight both rows to do it so if i leave the selected cell on product id 3 it will upate only that row. is there a way to update the rows that have been changed when not highlighted.

for (int i = 0; i < dataGridView1.SelectedCells.Count; i++) {
int selectedrowindex = dataGridView1.SelectedCells[i].RowIndex;
DataGridViewRow selectedRow = dataGridView1.Rows[selectedrowindex];
string a = Convert.ToString(selectedRow.Cells["Quantity"].Value);
string b = Convert.ToString(selectedRow.Cells["ProductID"].Value);
//update to database.....
}