Hello
Do you mean the desktop background slideshow?
If so, this needs to be done in registry editor.
Type regedit in the search on the taskbar and press enter to launch Registry Editor, then go to this path:
HKEY_CURRENT_USER\Control Panel\Personalization\Desktop Slideshow
Double click on the Interval. In the pop up, change the base to Decimal. The unit here is milliseconds. If you tell me what interval you want to set to, I can help you convert it and tell you what number to put in there.