How to change file permissions to edit footer

Hi there,

I would like to make some edits to the header and footer of my website, https://cpd.berkeley.edu/, but when I go in to try to make them, it says: *You need to make this file writable before you can save your changes. See the Codex
<https://codex.www.ads-software.com/Changing_File_Permissions&gt; for more
information.*

Does anyone know how I can make the file writable so that I can edit it? I
would like to add an official seal, logo, and contact information in the footer.

Thanks!