You need simply wait for finish of update process then the message should disappear.
Did you read our documentation?
Follow the steps below to update theme through FTP:
Download the zipped package from ThemeForest and extract to your desktop.
In the extracted package, you will find archive XStore_Theme_v.x.x.zip; the WordPress theme.
Extract the zip file and upload the extracted XStore folder to /wp-content/themes/ folder on your server with overwriting.
Check to see if you are using the latest version of the XStore Core plugin. Update this plugin if your version is not the most recent.