Hi Haoyu Ren,
Yes, it is possible to change the default value of an attribute by microflow.
If I am not wrong, in your case you are trying to change the value of Enumeration Attribute from Select Boards Entity. everything is fine except one thing, you are passing a String Value to enum value in the change object ( if my Assumption on ur case is right). You have two ways to set the value, Create an EnumVariable $SPARQlQuery which matches the Enum attribute in the entity, and inside the create Variable put an if-else condition on the string Value NewSPARQlQuery and match set the matching enum value and then Set the $SPARQlQuery in the change object od directly place the if-else logic in the change object.
I would prefer first on for easy understanding.
Let me know if this helps. Good luck !!