Yes. Patients can always change their minds about DNR orders at any time. If you change your mind, advise your doctor of that fact. The doctor must legally cancel and remove the medical order from the record.
Suppose you change your mind about a living will, medical power of attorney, or advance healthcare directive. In that case, you should advise the person you have designated as your representative in those documents that their power has been canceled.
The best practice is to retrieve the document and physically destroy it. Some states require the cancellation of a living will or power of attorney in writing, which is also a good idea.