Step 1: Download your Lightroom Presets from 123presets. The presets will come in a .zip file.
Step 2: Navigate to the Develop Module in Lightroom and click on File>Import Develop Profiles and Presets (see image below).
Step 3: Locate the downloaded zipped preset file or the folder with unzipped XMP files.
Step 4: Open Presets Panel, expand presets folder, and click a preset to apply it.
DOESN'T WORK? TRY ANOTHER METHOD
Launch Lightroom Classic, click the small "+" on the Presets Panel, and select "Import Presets." Locate the downloaded zipped preset file, and the presets will install automatically!
Ensure you are in the Develop mode to view the Presets tab.