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 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
If via Wifi - Disconnect all other Devices using the Wifi - want the Maximum Bandwidth only for this computer
There are several ways to download the Big Sur 11.5 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 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
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.
Any of the above will interfere with the download and installation process