Update value in a collection

I have a DataGrid with an item source as an observable collection. At button click on of the value in a column of the current row gets increased by 1.  So I went to solve it this way;

ItemRow currentRow = (ItemRow)myDataGrid.SelectedItem;
newQty = currentRow.qty + 1;
newExtPrice = newQty * currentRow.price;

myCollection.Remove(currentRow);
myCollection.Add(new ItemRow { itemid = currentRow.itemid, qty = newQty });

The problem is that the DataGrid view order gets changed.  So for example say there were 4 rows in the grid and the user clicks on the first row, that row is removed and placed at the bottom.  What is the better way to handle this?