You would like to change how the Delete button works.

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By default, tapping the red Delete button in the Message Area clears the entire Message.

You can change this behaviour, so that one tap deletes the last word and a second tap clears the entire Message:

1. If the top toolbar is hidden, tap the black line above the Message Area. 
2. Tap the Settings (cog) icon in the toolbar, choose Message Area, then Delete Button, and then select Delete last word first.