That’s why we encourage you to make child theme instead. ??
First, make a backup: https://codex.www.ads-software.com/WordPress_Backups
Then, follow this guide: https://codex.www.ads-software.com/Upgrading_WordPress_Extended#Upgrading_Across_Multiple_Versions
Finally, edit the header of your custom Twenty Ten’s style.css file, which you saved in the backup, to:
/*
Theme Name: Twenty Ten Custom
Theme URI: https://www.ads-software.com/themes/twentyten/
Description: Custom Twenty Ten
Author: the WordPress team
Author URI: https://www.ads-software.com/
Version: 2.0
License: GNU General Public License v2 or later
License URI: https://www.gnu.org/licenses/gpl-2.0.html
Tags: black, blue, white, two-columns, fixed-layout, custom-header, custom-background, threaded-comments, sticky-post, translation-ready, microformats, rtl-language-support, editor-style, custom-menu, flexible-header, featured-images, featured-image-header
Text Domain: twentyten-custom
*/
Then, change the folder’s name to “twentyten-custom” and upload it to /wp-content/themes/
Finally, navigate to Appearance -> Themes in your blog’s Dashboard and activate Twenty Ten Custom.
Eventually, you’ll want to turn just the changes into a child theme of Twenty Ten so that you can keep improvements/updates to the parent Twenty Ten theme without losing your customizations or using an entirely separate theme:
https://codex.www.ads-software.com/Child_Themes
https://themeshaper.com/modify-wordpress-themes/