Open a terminal and just execute:
sudo chmod -R 775 /Applications/WebStorm.app/Contents
Close the window and check for updates again. The error should disappear and you should be able to update Webstorm
Open a terminal and just execute:
sudo chmod -R 775 /Applications/WebStorm.app/Contents
Close the window and check for updates again. The error should disappear and you should be able to update Webstorm