How to change NaNs in dataframe to 0?

Question

I joined two data frames, and now have values in columns that say "NaN." I am trying to convert those cells to 0.

I have tried this:

df = final_data_frame.apply(to_numeric, errors='coerce').fillna(0, downcast='infer')

but I get the following error:

NameError: name 'to_numeric' is not defined