how do i modify route=checkout/success

i can't find the php file

The ?route= appends to index.php itself. The success.* files (.php and .tpl) are in the /catalog/ tree. All of the files in the /catalog/ tree are in familes by name, separated among model/, controller/, language/, and /view/theme/[which]/template/ files. The files are .php except the template .tpl files.

If it's words you want to change, look in the language files

Some of the words such as for dropdowns and certain other contexts are either not even in the language .php files or are there plus elsewhere, among the other one to two .php files and the .tpl file, in each name-family set of three to four files.