Short answer

You can modify only active Good Till Triggered (GTT) orders. Once your GTT order triggers, you cannot modify it.

On Arrow app

Tap on Orders and then on GTT.

Tap on the instrument and then on Modify.

Make the required changes and swipe Modify GTT.

On Arrow web

Click on Orders and then on GTT.

Click on Options and then on edit.

Make the required changes and Click on Modify.