How to upgrade an older Mac Mini to newest OS and newest firmware? I am having trouble to download the latest OS Update. 
When I try to install the update using native mac features I get an error like

update couldn't be verified. While loading it was probably damaged...


So I tried command line.

softwareupdate -i -a


Result:

    Software Update Tool
Copyright 2002-2010 Apple

2018-05-26 20:12:49.918 softwareupdate[4498:4207] PackageKit: Missing bundle path, skipping: &lt;bundle id="com.apple.XsanAdmin"&gt;&lt;/bundle&gt;
2018-05-26 20:12:49.963 softwareupdate[4498:4207] PackageKit: Missing bundle path, skipping: &lt;bundle id="com.apple.airport.airportutility"&gt;&lt;/bundle&gt;

Downloading Mac OS X Update 
Package failed: The operation couldn’t be completed. (SUSessionErrorDomain error 4.)


My Mac Mini has the following specs:
2 GB DDR 2 Memory
enough HDD Space
1,83 GHZ x2

Here all specs in german:

  Modellname:   Mac mini
  Modell-Identifizierung:   Macmini2,1
  Prozessortyp: Intel Core 2 Duo
  Prozessorgeschwindigkeit: 1,83 GHz
  Anzahl der Prozessoren:   1
  Gesamtanzahl der Kerne:   2
  L2-Cache: 2 MB
  Speicher: 2 GB
  Busgeschwindigkeit:   667 MHz
  Boot-ROM-Version: MM21.009A.B00
  SMC-Version (System): 1.19f0
  Seriennummer (System):    YM7350C8YL1
  Hardware-UUID:    00000000-0000-1000-8000-0016CBABF5B5


I am currently running on Version 10.7.4

What are my options?
Thank you in advance
Unfortunately, this Mac Mini can only be upgraded to macOS version 10.7.5.

Source: MacTracker.app on both iOS and macOS. 

This Apple Community post also verifies this.