Since a pending order is placed for a future trade, you can modify or cancel it completely.
To modify or cancel a pending order:
Locate the pending order on your MT5 trade window.
Right-click on Modify or Cancel.
In the new window, you can change the parameters you wish to alter and then click Modify. You can also choose to cancel the order by clicking Delete.
Another way to cancel the pending order is to simply click the X button on the far right of the screen next to the pending order.
Keep in mind that a pending order will also be canceled automatically once the expiration date set by you is reached.