Go to your name in the top right corner, select "Profile" - "Notifications". There, you will find a list of all notification events sorted by subject.
Choose according to your needs, and click on "Save" to change your changes.
Please note that you can only change the notification settings for your own user.
Can I turn on/off a recipients notifications?
No, but they can do it themselves by clicking the Settings cogwheel in the right bottom corner of their contract view.
However, if they turn off notifications they will not receive any information about updates in the contract.