Unable to update to Mac OS 11.2.3?

I have problems updating my MacBook Pro (2016 edition) to the latest version 11.2.3 Big Sur

First of all, I am running on 11.2:

sw_vers
ProductName:    macOS
ProductVersion: 11.2
BuildVersion:   20D64

Now my System tools will announce that the 11.2.3-update is ready and pester me to install it. When I click "Update", it will start downloading a 2 MB (yes, MEGAbyte) update in an extremely slow speed. This takes around 10 minutes. Then, hooray, an update has been found: "11.2.3", weighing 2,5 GB. Downloading this then takes a day. A whole day! If not more!

Its not my connection, which is with approx 10MBit good enough I would say. In the meantime (i.e. in parallel), I updated IntelliJ and my entire MS Office without problems. Also, I have plenty of disk space free.

Now, after downloading the 2,5GB, the Mac will ask to restart, then announce that "there's an update". Guess which one it is? "Big Sur 11.2.3".

How to get rid of this and install 11.2.3 for real?

I've had success with doing one or more of the following:

Content Caching

• Set up Content Caching

Go to System Preferences > Sharing. Turning Content Caching. Restarting the Mac. Apparently Content Caching reduces bandwidth usage and speeds up installation on supported devices by storing software updates on the computer. We made sure that the Cache Size was unlimited - click on Options. And we also made sure that the option selected beside Cache was All Content.

• Reboot
• Run the Software Update check again
• Once downloaded, go through the normal steps of upgrading macOS

Official Standalone Installer

• If content caching fails, download the installer from the Mac App Store
  • Call Spotlight (Cmd-space) and type "app store," then hit return
  • Search for "big sur" at the top left hand side
  • Select VIEW on the top result of macOS Big Sur
  • Then choose GET
• Once downloaded, go through the normal steps of upgrading macOS

Terminal

First-party software can be updated from the terminal using softwareupdate.

• List available updates: softwareupdate -l
• Install all available updates: sudo softwareupdate -ia
• As of macOS Catalina 10.15, fetch the full installer: softwareupdate --fetch-full-installer --full-installer-version 11.2.3
• Once downloaded, go through the normal steps of upgrading macOS

Semi-official Standalone Installer

Another route is pulling the standalone installer from Apple's CDN via an open-source Python script called installinstallmacos.py.

Install git

• If git isn't installed or raises a xcrun: error, install/update git
  • Homebrew install: brew install git
  • Manual download and install

Setup script

# create git directory and download git repo
mkdir -p ~/git && cd $_
git clone https://github.com/munki/macadmin-scripts.git

# change to the new directory
cd ~/git/macadmin-scripts/

# make executable (may not be necessary) then run the script
chmod +x installinstallmacos.py
sudo ./installinstallmacos.py

Choose OS and download installer (output edited for brevity)

installinstallmacos.py - get macOS installers from the Apple software catalog

 #      ProductID    Version    Build   Post Date  Title
 1      001-15219    10.15.5  19F2200  2020-06-15  macOS Catalina
 2      001-68446    10.15.7    19H15  2020-11-11  macOS Catalina
 3      001-04366    10.15.4  19E2269  2020-05-04  macOS Catalina
 4      071-08935     11.2.2    20D80  2021-03-01  macOS Big Sur
 5      061-86291    10.15.3  19D2064  2020-03-23  macOS Catalina
 6      041-91758    10.13.6    17G66  2019-10-19  macOS High Sierra
 7      041-88800    10.14.4  18E2034  2019-10-23  macOS Mojave
 8      071-14766     11.2.3    20D91  2021-03-08  macOS Big Sur

Choose a product to download (1-16): 8

• Once downloaded, it will be archived in a DMG
• Open the DMG and go through the normal steps of upgrading macOS