Replace existing column with merged column and rename

I am merging two dataframes. The first dataframe looks like this:

A       B        C           Dparty1  asset1   product1    Buyparty1  asset1   product2    Sellparty2  asset2   product1    Buyparty2  asset2   product2    Sell

The second dataframe looks like this:

A       B        Dparty1  asset1   Buyparty1  asset1   Sellparty2  asset2   Buyparty2  asset2   Sell

I merge the dataframes like this:

df2 = df.merge(df1, on=['A', 'B', 'D'])

which returns:

A       B        D      Cparty1  asset1   Buy    product1party1  asset1   Sell   product2party2  asset2   Buy    product1party2  asset2   Sell   product2

I need to replace the values in column 'B' with the values in column 'C' while keeping the same name 'B'.

A       B          D      party1  product1   Buy    party1  product2   Sell   party2  product1   Buy    party2  product2   Sell   

Any suggestions on how to achieve this would help.