You have to download the theme from wordpress.org and upload only footer.php file on your server
You can also reinstall theme and but i suggest to try first method
When I download the theme and only upload the footer.php text I get this message:
“Unable to communicate back with site to check for fatal errors, so the PHP change was reverted. You will need to upload your PHP file change by some other means, such as by using SFTP.”
You must upload file, not only the text, replace the footer.php file,
there is various ways to do this, form a hosting panel file browser, from FileZilla and so on
if you have problem with this question get in touch with your hosting support