where to paste source code

Direction is:
Go to site FTP or through HTML editor and paste code above </body>
I spent 1/2 day on it, called hosting company, nobody helped me
to find the place. I’m afraid to mess-up something.
Please somebody tell me where is site’s html is, at the bottom should be place to post the code just above “body”
I know the visual/html where posts are created, of course, but this should be really something that then could be transferred into via FTP into public_html.
Thank you so much for help!!!!!
Regards,
M