Fatal Error when trying to do update

I’m trying to update my new install to Piwik 2.15.0. It tells me that since it’s a big update it’s best to do the update from my command line. When I run the update from the command line, I get “PHP Fatal error: Class ‘Piwik_Plugin’ not found in /var/www/clients/client0/web3/web/watch/plugins/DoNotTrack/DoNotTrack.php on line 20”.
What is going wrong??
Any help - especially the “dummies” version - is greatly appreciated.

Thanks in advance,

Delete the folder /plugins/DoNotTrack and try again