Stone of Jacob. Before you modify any files, make sure you read and understand the Zen Template overide system. Very good information is available in the FAQ/ Tutorial section. All new Zenners should understand this before trying to change things in files.
If you don't, upgrading to new versions, and future changes can be a nightmare.
If you do understand the template system, all fear should evaporate, because if you open a file and get it really messed up, you can always go back to the original file so that your site works, and try to make your mods again.