You can update a WordPress theme to its latest version through FTP. This is necessary for some premium and custom themes that can't be updated in WordPress. You'll need to download the files for your theme to your local computer before starting this process.

Connect to your hosting with FTP.

Navigate to the /wp-content/themes folder for your WordPress site.

Upload your theme to this folder.

If your theme is zipped, you will need to unzip it.

Note: If you don't have a way to unzip the file on your hosting account, you'll need to unzip the file on your local computer, and then upload the unzipped folder of files.

If your theme has a custom stylesheet, also known as a child theme, repeat the process for this as well.