Reading the excel file is quite straightforward. You can use OleDbConnection to create a connection to it and then read the data using OleDbCommand. For example, have a look at
Read Excel File into DataSet in ASP.NET Using C#[
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After you have got the data into the datatable, loop through the rows and create the output. In pseudo code, a very simple approach could be
for each row in excel data table
open file(row[0]) for apppend
replace string(row[0]) with ...
close file
next row
Of course that code unnecessarily opens and closes the same file if the file is the same between several rows so you can optimize that if needed.