The Pandas library will give you the tools to replace the nan values.

Fillna

The .fillna() method will replace all NaN values in contained in a pandas.Series ou pandas.DataFrame.

The method takes multiple arguments such as

Value

The value you want to replace the Na with.

Method

The method of fill. Can be forward fill as "ffill", backward fill as "bfill".

Axis

The axis you want to perform the operation on, it works only when given a DataFrame.

An example on a Serie

# My column is the column in which 
# we want to replace our NaN
df["my_column"] = df["my_column"].fillna(0)

An example on a DataFrame

# Here we apply the method on a dataframe itself
df = df.fillna(0) # Here axis 1 means columns

Here you are, you know how to replace your NaN by a specific value !

If you want to learn more about Backfill or Forward Fill, you can check the links below