You can use this guide to upgrade the 2.5-inch hard drive in your Late-2014 Mac Mini to a larger version, or replace the hard drive with a high performance solid state drive.
Mac Mini Late-2014 HDD/SSD Replacement Guide (MacMini7,1)
You can use this guide to upgrade the 2.5-inch hard drive in your Late-2014 Mac Mini to a larger version, or replace the hard drive with a high performance solid state drive.