Yes, you can modify and cancel any pending GTT order, but once it is executed, you will not be able to modify it.
Follow these steps to modify your pending GTT orders:

1. Open the Share.Market app
2. Navigate to Portfolio > Orders and tap GTT
3. Select the specific pending GTT order
4. Tap MODIFY
5. Enter the Trigger Price and Quantity
   • Select your price type: Limit or Market
6. If you select Limit, enter your specific limit price
7. Review your order details and tap MODIFY

Note: Tap More to set Validity after trigger and Disclosed quantity.

Follow these steps to cancel your pending or open GTT orders:

1. Open the Share.Market app
2. Go to Portfolio > Orders and tap GTT
3. Select the specific pending GTT order 
4. Tap CANCEL and confirm the cancellation