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.
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.