Big Sur Update

Agree totally.

Thusly, the other options outlined earlier. Apple has more work to be done on the delivery process. Since they have apparently discontinued, in Big Sur, to offer the Standalone updates like in previous versions of macOS - we have to devise other methods to achieve the same results.

Kind of keeps us on our toes :-)

If this update is for Big Sur 11.5 - the error message can be misleading. There are factors like poor Wifi connection, slow wifi connection, too many other people downloading at the same time and Apple Severs are overloaded, Software on the computer interfering with the download process, running behind a VPN and AV Software Installed and or / if via wifi too many Other devices using the wifi at the same time reducing the Bandwidth to you computer. Best Results are via Ethernet Cable to Router

There are several ways to download the Big Sur 11.5.1 update. 

1 - Connect to Router via Ethernet connection and restart the computer in Safe Mode and then open the Apple Apps Store and get the Full Installer of Big Sur. It will be the 11.5.1 version and once downloaded run the installer from the Applications folder.

2 - Resart the computer in Recovery Mode ( INTEL Command + r ) and choose Reinstall macOS. It too will download the full Installer and commence the installation. For M1 CPU the Recovery Mode is Here

3 - This method may seem unnecessary but sometimes the Router Restart clears Network issues from ISP to Apple Servers

Shutdown Router. Shutdown computer and disconnect all External Drives. Restart Router. Connect to Router via Ethernet Cable. If via Wifi - Disconnect all other Devices using the Wifi - want the Maximum Bandwidth only for this computer. Restart computer in  Safe Mode. It does a Repair Disk, Clear System Cache files and only load Apple Software, extensions and Fonts. Will load slowly - Normal. Once Logged in - open Apple Apps Store and  Get the Full Version of Big Sur. It will take some time and several pauses will occur - normal. Countdown clock may say 15 minutes but in Real time may be 30 minutes. Same goes for clock at 10 minutes could be 20 minutes in Real time. Eventually , Full Installer will download and a New Application in Applications Folder will present called Install Big Sur. Stop there and perform a Time Machine Backup. If desired - have an External Drive - Copy & Paste the Install Big Sur Application to external drive for future usage - making a Bootable Installer. Now run the Installer.

Have found this method more reliable than the Software Update which is very iffy at best

Are there any AntiVirus, Disk Cleaner, Optimizers, Defrag, VPNs etc installed which should be removed as per Developers Instructions. They are useless, unneeded, cause havoc and interfere with the normal operation of the OS and may even Corrupt the OS requiring a Reinstallation.

Related: My MacBook Air M1 is UNable to access the 11.5.1 update in Software Update, three days after its release date. Seeing as I'm in AppleSeed, this is extremely odd. Checking around the net, there have been other reports of 11.5.1 locking up Macs, for example comments over at 9to5 Mac. My impression is that Apple has pulled access to the update for certain Mac models. I'll be contacting Apple Support to press them for an explanation.

User wrote " Apparently the software update for this 11.5.1 is useless. " in what way is it useless ? Only Apple knows exactly what was not correct in 11.5 and was subsequently Fixed in 11.5.1.

None of us have specific knowledge that can be presented as proof and to support such as claim.

Agree to disagree with respect.

The 11.5.1 update via System Preferences worked just fine on my unit M1 MBA. That does not mean it work file for all others - agree.

As for the usefulness of the 11.5.1 - still waiting for documented proof that supports the original claim that it is useless.

As for whining - recheck from whom that originated from. But, I am used to these types of comments. As Useful as the comments about 11.5.1 being not useful.

I worked with Apple to sort out the lack of access to the update via Software Update. Neither of us discovered the problem. I ended up going into Recover Mode, running Disk Utility (no problems found), then doing a reinstall. The result was full 11.5.1. Annoying.

"User wrote " Apparently the software update for this 11.5.1 is useless. " in what way is it useless"

Read about it here, and please stop your whining: https://support.apple.com/en-us/HT212622

If you download and run the full installer from the App Store app, it will install version 11.5.1, the current version.

Keep in mind that the time remaining indicators are notoriously not accurate. I have routinely seen 1 minute remaining lasting for 15 minutes sometimes.

Welcome and thank you too