There are a few options here:
https://www.photo.gallery/docs/help/updating/
Since your panel doesn't have the update option, I recommend you try updater script first:
1. Download
x3_updater.php.
2. Upload into your X3 directory.
3. Make sure you are logged in to panel, and the access the uploaded x3_updater.php script from browser.
If the above doesn't work, you would have to "Update X3 manually by FTP" (see the top link).