To modify an existing order, navigate to the trade tab and click on it.
This shows all the trades you have placed, including the sizes, stop loss, take profit, and entry price for every trade.
In rare cases where the trade tab is nowhere to be found, click on CTRL + T to open it.
From here on:
It is worth noting that the modify option on the modify window won't be available until you make a change on either the stake profit, stop loss or both.