Preview can only be upgraded when you upgrade to a newer OS.
Apple does not retrospectively upgrade its Camera RAW database.
So if your particular model of Leica camera was introduced after High Sierra
then the camera raw database will never be updated to include your camera.
It is the camera raw database that allows for the viewing of raw files in Finder, Preview,
Quicklook and Photos.
Here is a list of the cameras supported in High Sierra,
Digital camera RAW formats supported by iOS 11 and macOS High Sierra – Apple Support
If you want to upgrade your OS there are two options if your mac meets the criteria,
open your Safari browser and click on the link below, Firefox will not work.
How to upgrade to macOS Mojave - Apple Support
How to upgrade to macOS Catalina – Apple Support
Digital camera RAW formats supported by iOS 12 and macOS Mojave – Apple Support
Digital camera RAW formats supported by iOS 13, iPadOS 13 and macOS Catalina – Apple Support
If your mac cannot upgrade further then you can download the free Adobe DNG converter,
this converts the Leicas RWL files to the .dng format (digital negative) which will be viewable in
Finder, Preview, Quicklook and Photos.
https://helpx.adobe.com/uk/photoshop/digital-negative.html
Did your Leica camera not come with its own RAW file software.