1. Activate the developer mode

In Odoo 10 and later you can do this from the "Settings" tab, in Odoo 9 and older you would have to click on the user in the top right corner and then "About" - the "Activate the developer mode" link will be there.

2. Navigate to the view you want to change

For example: if you want to change the Privacy default setting in the Calendar click on the Calendar and then create a sample meeting. Now change the setting you the want you want to set as default.

3. Click on the debugger icon and then "Set Defaults"

If you can't see the debugger button try to refresh the page or jump to point 1. Set Defaults position in different Odoo versions may vary but it is there.

4. Set defaults

From the drop-down menu, you can choose currently selected settings(from point 2). You can choose if the new default will apply to all Odoo users or just for you. This might be useful as users might want to have different defaults. Save defaults and it is done.