Thanks for asking.
Unfortunately, in your case you will have to first update X Theme manually. Reason being the purchase code won't work as we have retired the validation process that required API.
So to begin with, I suggest you to first Install and activate child theme (if you haven't done that) and move all the changes you have made inside parent theme to child theme. If you haven't made any changes in parent theme, then no need to install child theme.
Here are the resources that you can use to download and setup child theme.
After that, please carefully review the steps mentioned in our theme and plugin update guide. Kindly give special attention to Manual Updates section.