won't download Big Sur -- time's out

If you're using the System/Software Update preference pane to download try going to the App Store, search for Big Sur, download the full file updater and install. It's often more successful than the preference pane. 

If you have problems with installing after downloading boot into Safe Mode by booting with the Shift key held down and install from there. 

NOTE: Safe Mode boot can take up to 10 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

The method by @ Old Toad is the preferred method and usually works very well - so try this before attempting this method which is longer to execute.

Restart Router and then Shutdown Computer and connect via Ethernet connection. Restart computer in Safe Mode - Shift - Key immediately at startup. It does a Repair Disk, clears cache files and load only Apple Software - load slowly - Normal. Once in Open Apple Apps Store and find Big Sur and download. Try not to let computer go to sleep mode or Screen Save to activate. Be patient as one knows it Big Sur is 12.18 GB size. There are often periods where it seems to pause the download , normal be patient as countdown clock may say 15 minutes but in Real Time could be 30 - 45 minutes. This can and will happen several time in the process. If all goes well. it will successfully download and attempt to Launch. Do NOT allow it to start - Quit the Installer. Check Applications Folder for new App called Install Bug Sur. Make a copy to External Drive - future usage. Now, restart again in Safe Mode - then launch the Installer.

Welcome and Thank you too.

Do come back to theses Forums if or when addition questions arise. There are many Contributors will to assist.