how to rewrite in same excel sheet after running same program again with different input?

I want to be able to write data to an excel spreadsheet after running a program. I know how to do this once, but I want to be able to run the program many times and add the data to excel each time, without overwriting the previous data in the spread sheet. In my program, after scanning a barcode i display decoded number in A1 block of excel sheet. Now i want to scan another barcode by running same program again and want to display new decoded number in A2 block of same excel sheet and so on. can you please suggest me some code that will help me.