Click on "edit" icon

You could now edit the price by changing any field value (Subtotal, Shipping, Taxes, Total or Partically Paid).

Click "Save" to automatically add up the total.

If you don't want the total to be automatically add up. You could override this feature by clicking "more option" and click "Save without adding up the values".

Why you need this override feature? Some order might not need taxes.