The folder you need to check is C:\ProgramData\3CX\Data\Http\Interface\ivr (not Rollback\AppData\...).
Anyway, I'm not sure if you're going to be able to do it that way, because there is much code there already compiled to a DLL, so even if you change the sources, the code will run from the DLL. So, give it a try if you want, maybe changing VXML files works, but changes to CS files will not work. I don't know what file you need to change. The clue I can give you is that everything starts from ivr_start.aspx (code behind Default.aspx.cs).