Processing file according to excel data.

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#[^]

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 ...
   // other replacements if any
   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.