Hi there,
Download X theme from the dashboard and extract x.zip file. Then using FTP, go to this folder wp-content/themes/. Delete existing x folder. Then upload the new x theme folder that you have extracted. This might help:https://community.theme.co/kb/updating-your-theme-and-plugins/
It is always recommended to backup your site. The content will still be there since it is save in the database.
Hope this helps.