There is no keyboard shortcut assigned to scroll through Presets Adobe Photoshop Lightroom Help | Keyboard shortcuts
However you can hover over your mouse on each preset and the Navigator will display a preview of the Preset effect.
You could request for such feature here Photoshop Family Customer Community
As Akash pointed out, there is no keyboard shortcut for scrolling through presets. Please add your vote and opinion to this topic in the official Adobe feedback forum: Lightroom: Ability to browse the presets and see its effects with the up and down keys. Meanwhile, you might take a look at the Any Preset plugin, which lets you preview presets at full size (not just in the tiny Navigator window) using the cursor keys.