This question really misses out on some major details that could help us with your specific problem but he, here is a general template that might help you out a bit more:

import pandas as pd

# read data

df = pd.read_csv("data.csv")

# replace nan with a value

df\["column_name"\] = df\["column_name"\].fillna("Unknown")

# Check result

print(df\["column_name"\].isna().sum())