Firstly: settings:
1- Login to the control panel
2- from the side list, select (settings)> then (plugins settings) as the picture below:
3- the settings list will show up, choose (stream settings) as the picture below:
The below page appears:
Newsletter cycle period in days: select the day when you want the newsletter is sent.
Max items per section in newsletter: set the maximum number of the section contents that are sent per time.