Changing system_instructions after model has been allready initialised ? help

Gemini API
,
1

how do i change system_instructions after model is initialised?

for example first initialisaiton:

knowledge_integration_model = genai.GenerativeModel(
       system_instruction=knowledge_integration_instruction,
       model_name=MODEL_NAME,
       tools=knowledge_integration_tools,
       safety_settings={"HARASSMENT": "block_none"}).start_chat(history=[])

after that at some point i would like to send_message(prompt, ??? system instruction???)