Please follow the steps below to cancel an open limit order in TD Ameritrade Mobile app.
Open the TD Ameritrade app on your iPhone and log in using your user ID and password.
Once you have logged in, tap on the Accounts icon which is found at the bottom of the screen.
On the top of the screen, you will see several tabs. Tap on the tab which says Orders.
Once you tap on the Orders tab, you should be able to see all your open orders. Tap on the particular order you want to cancel.
Once you have tapped your particular order, you will see two options on the screen: Edit order and Cancel order. Tap on the tab which says Cancel order at the bottom.
You will be asked to confirm whether you want to cancel that particular order. If you would still like to cancel the order, tap on Yes.