Can I update my mini Mac OS software to a later version 10.15

Mac mini late 2014 is compatible with Monterey, the current macOS version.

Upgrade to macOS Monterey - Official Apple Support

You will want to confirm all of your apps are 64-bit or can be updated or upgraded or replaced with 64-bit versions, that your printer and scanner vendors have drivers available for Monterey. You can use the Go64 tool to find the 32-bit apps that will need to be replaced or upgraded or retired. This list of apps that will need upgrades to 64-bit apps includes Microsoft Office 2011 and earlier, which are 32-bit apps, and which will not work on macOS Catalina and newer—and all of which means acquiring newer Office version, or migrating to one of the available alternatives.

Have a backup prior to upgrade

Hi,

Per your question, yes, you can upgrade to 10.15: How to get old versions of macOS - Apple Support