Testing checksum by modifying a file

A program in C# which copy a file or whole folders to another folder is made and in that application checksum SHA-512 has been used to verify that input and output of the copy process is/are identical, the program works fine but I need to test the whole program and especially test or verify the checksum. how can I, give the program an input e.g. a file and in the process modify the file somehow in order to see that checksum detect that error? thanks for your suggestions