Python Pandas update a dataframe value from another dataframe

Using DataFrame.update, which aligns on indices (https://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.update.html):

>>> df1.set_index('Code', inplace=True)
>>> df1.update(df2.set_index('Code'))
>>> df1.reset_index()  # to recover the initial structure

   Code      Name   Value
0     1  Company1   200.0
1     2  Company2  1000.0
2     3  Company3   400.0

You can using concat + drop_duplicates which updates the common rows and adds the new rows in df2

pd.concat([df1,df2]).drop_duplicates(['Code','Name'],keep='last').sort_values('Code')
Out[1280]: 
   Code      Name  Value
0     1  Company1    200
0     2  Company2   1000
2     3  Company3    400

Update due to below comments

df1.set_index(['Code', 'Name'], inplace=True)

df1.update(df2.set_index(['Code', 'Name']))

df1.reset_index(drop=True, inplace=True)