Currently, the content in the Message box cannot be edited, apart from deleting the words from the end, one at a time, or clearing the Message box content completely. There is no provision to place the cursor in the middle of the sentence being formed, to edit it in the middle.
You can tap the delete Delete button to delete the words one by one, or you can use Clear button to clear the Message box content completely. 
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How to clear Message box content?
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