Unable to update to Mac OS 11.2.3?
I've had success with doing one or more of the following:
Content Caching

Set up Content Caching


Go to System Preferences &gt; 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 &quot;app store,&quot; then hit return
Search for &quot;big sur&quot; 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 &amp;&amp; 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