macOS Big Sur update is stuck

If this update is for Big Sur 11.5.1 - 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 including AntiVirus Software , running behind a VPN 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 .1update. 

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 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.

Good points from @woodmeister50 and worth listing to them

Do you have a Rescue Plan In-Place. This comprises having 3 Backups using 2 methods and 1 Off Site incase of Theft, Loss or Natural Disasters. A Time Machine Backup  is very useful and can be used to Revert to Previous Working macOS. For addition purposes - two Tested Bootable Clones each to separate external drives. This is insurance incase the upgrades goes sideways. At least one Tested Clone and / or Time Machine Backup should be Off - Site

Can not speak about the reliability and / or speed of the ISP suppling your Internet Connection beyond a comparison.

My ISP is only a 50 Mb Download and 10 MB upload speed - for the moment and going to upgrade with ISP to Fibre Optic shortly. This ISP on not sharing the Bandwidth across all Others in my area but dedicated to my connection.

The Big Sur Full Installer ( 12.24 GB ) from Apple Apps Store over Wifi ( yes Wifi ) took about 20 minutes and this was during daylight hours.

It is very irregular for the update to take days to complete, if at all.

Would suggest speaking Directly with your ISP and discuss this issue unless there is an ISP CAP on downloads / usage connected to your service ??

Jarslamares wrote:

Actually, there is no error message - nothing really happens. On my MacBook Pro it took forever to download but it's now fine.

...

I am not quite willing to reinstall OS X as you suggest since I guess it will men a clean install and then I have to reinstall all my apps and it takes a lot of time.....

False, re-installing the OS will only replace this items of the OS and will not touch any of your apps or data. Of course anything can happen which is why you should always have a backup. Storage devices are cheap these days so there is really no excuse not to have at least a Time Machine backup of your system. Trusting your digital life to a single device is an accident waiting to happen.

Fusion Drive ?? Fusion Drive are ok for storage but unless the SSD is large - not too fast and especially on upgrades to more recent versions of macOS.