How to call an assistant by id and insert a file to learn from it

what should i update in this lines of code to call an assistant by id instead of calling all assistants related to the API, and how to add an excel file to answer questions about it .
code :
def config(client):
my_assistants = client.beta.assistants.list(
assistants =
assistants_dict = {“Create Assistant”: “create-assistant”}
for assistant in assistants:
assistants_dict[] =
assistant_option = st.sidebar.selectbox(“Select Assistant”, options=list(assistants_dict.keys()))
return assistants_dict[assistant_option]