How do I transfer a slideshow from my iPhone to a laptop?

Created in what app? You probably can just sync your iPhone to your laptop (via Finder on a Mac or iTunes or Apple Devices on a WIndows system) and transfer the file or, if the app supports iCloud, sync via that service. But without knowing what app you're using and what laptop you have, we can't be more specific.

Regards.

Then start here:

Use iTunes to sync your iPhone, iPad, or iPod with your computer - Apple Support

or if you have the Apple Devices app:

Sync your Windows device and iPhone, iPad, or iPod - Apple Support

Regards.