Hello,
1. Note that you can directly customize the "static" content of your email by edit your translation file via "Hikashop->System->Configuration->Languages->'YourLanguageFile'" :
www.hikashop.com/download/languages.html#translate
2. If you really want to change the code of your email without the business version, you'll have to change the code of the files on the "media\com_hikashop\mail" folder, but note that you'll have to save your changes because if your update/re-install your Hikashop you'll loose your customization.