SubQuestionQueryEngine uses prompt for generating sub questions , if you want to change the prompt for generating the sub questions you can do it this way:
CHeck both the generator code:
https://github.com/run-llama/llama_index/tree/main/llama-index-integrations/question_genThey have a default prompt that they use to generate queries, modify that and create the object of question generator and pass it in the subquestion engine:
question_gen = OpenAIQuestionGenerator.from_defaults(prompt_template_str="ADD_HERE"...)
# add it to SubQ
query_engine = SubQuestionQueryEngine.from_defaults(
query_engine_tools=query_engine_tools,
use_async=True,
question_gen=question_gen
)