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You may change your mind on an open order and cancel it.
This is quite simple to do (contact your broker for details) but you will be responsible for prior fills. In other words, if 1,000 shares traded from your account in the time before you cancelled, this part of the trade is irreversible.
Changing orders is also an easy task, and the details will differ depending on your broker. You are still responsible for prior fills and you can not change the stock in question, or the volume. (If you want to change the stock or the volume, just cancel the one order and make an entirely new one).
You will be able to change the order between market and limit, adjust your price, or alter your duration.