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First - don't edit anything in the templates_c directory!  These are cached pre-compiled copies of the template, and are frequently cleared out.

Second - it's not advisable to edit the templates that ship with CiviCRM directly, because you'll have to apply your changes on every upgrade.

The best practice is to install civix and create an extension, but in your case, I don't see the harm of using a template override, and more specifically a ".extra.tpl" file.

This wiki page has all the information you need.  I would start by just creating a "custom templates" directory, and make sure you can successfully implement your change with a normal template override.  Then remove your override and reimplement using a .extra.tpl file, and your change should be OK through most upgrades.