Hello,
1) If you have already bought the theme you get lifetime access to theme updates and don’t need to buy the new license just to get the update.
2) Never update XStore core plugin without theme update. Always check the changelog and the necessary version of the core plugin for your theme version https://xstore.8theme.com/update-history/
3) Now you need either downgrade XStore core plugin or upgrade theme using auto-update (Dashboard > Updates) or manual FTP way.
4) If you need help with theme update provide us with Dashboard and FTP access to your site and well help you.
Regards