How can I convert '?' to NaN
You can also read the data initially by passing 
df = pd.read_csv('filename',na_values = '?')
It will automatically replace '?' to NaN
I have a dataset and there are missing values which are encoded as ?. My problem is how can I change the missing values, ?, to NaN? So I can drop any row with NaN. Can I just use .replace() ?