How to change certain values in a file

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http://www.unix.com – Hi all, i need help to replace certain values in a file. I need the script to check and match the ID and exNum1. if match, values in $3 (file2.txt) need to replace the value for 'START' (file1.txt) for each match. The sample structure is like this:- File1.txt Code: ID  P_6 START    235411 END    18763 // ID    P_10 START    631012 END    32814 // ID    P_133 START    389417 END    314124 // File2.txt Code: ex    21204    22151    P_6    S  &nbsp (HowTos)