Thanks for asking.
An easy option would be to use FTP software like Filezilla and under /wp-content/themes/ parent X Theme folder (if you haven't changed the folder name) should be X. You can delete other folder. To get started with Filezilla
You can also do following:
- Activate default WordPress Theme under Appearance > Themes.
- Delete both the parent X Theme.
- From members area, download latest version of X Theme.
- Under Appearance > Themes > Add New > Upload Theme select the file and install X Theme.
You can take a look at following article for more information.
Having said that, please make sure to take a complete backup of the website before making any changes. You can take a look at following article to backup website.