PHP settings not editable

In Plesk 12.5 web pro edition the PHP settings are not editable any more.
The settings are displayed as read only. How can I make the settings editable again?

Einstellungen zur Leistung

memory_limit 128M (Standard)
max_execution_time 300
max_input_time 600
post_max_size 256M
upload_max_filesize 256M
opcache.enable on (Standard)