awk command to replace columns from two different files

Hello, I have two files. The two files contains approx 2.5million lines. The first file contains around 20 columns and the 2nd file contains 3 columns. I would like to replace the whole column from the first file, 16th column with the data from the 2nd file -3rd column. So basically something like this 1st file 1st 2nd 3rd ...... 16th.... 20th a 1 x .......... 100 ... b 2 y .......... 200 ... c 3 z .......... 300 ... 2nd file 1st 2nd 3rd a 1 700 b 2 800 c 3 900 output needed: 1st 2nd 3rd ...... 16th.... 20th a 1 x .......... 700 ... b 2 y .......... 800 ... c 3 z .......... 900 ... any suggestions if I can use awk or sed ? Thank you.

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