Don't worry, just revert the patch that you installed using the command below, which will remove the changes made by the patch installed if you installed the patch through shell / command line.
sh patch-file-name.sh -R
You can also manually revert the patch as answered here
After that make sure to flush your cache completely by clearing the contents from var/cache/ directory
rm -rf var/cache/*
If you use version control , just revert the commit after this patch.
Besides these, if you really want to know how to find the core files modified by the patch, you can use these options below:
Comparing a complete Magento (core) install to stock