Hi,
I'm afraid that I'm not following you.
my developer modified the authorize.net plugin
Can you send me the authorize.net plugin so we can modify
The official Authorize.net plugin is include in HikaShop packages and you should see directly with your developer to have a re-install zip for that plugin.
More than that, we will recommend you (and your developer) to not modify official HikaShop plugin but to duplicate them ; by using another plugin name, you will be sure that you won't loose your modification with updates.
Because yes, when you update HikaShop, every official plugin is also updated.
Regards,