Hi Karisha,
Look in your file manager or via FTP and go to phpbb/templates/subSilver and then find the overall_header.tpl and overall_footer.tpl. These are the two files you need to edit to make changes to the footer and header across all pages of your forum.
Your directories may have different names, so instead of "phpbb" look in whichever folder/directory where you installed PHPBB. And instead of "subSilver" look in the folder that is named by the template you are currently using...
Hope that helps!