After setting the DMN Decision boxed expression type to any of the available options: Literal expression, Context, Decision Table, Relation, Function or Invocation.
You can't set back the expression type.
For example: If you intended to create a Function, but by mistake you select Decision Table instead, then you don't have a way to fix your mistake in the editor. You have to:

• Go back to the diagram, delete the decision node and draw a new one.
• Close the diagram with out saving changes and open it back.
• Use the xml to fix the property for the decision
  All of them makes the user lose time to fix an small click mistake.
  I would like to see functionality to re-select the expression type, specially if is a brand new decision with no body yet.