Is there another way to gat Big Sur? I get a "network connection lost" error when trying to download.

No problem. Here is what I think your problem has been. Clearly your download is not going to install. You need to check the Applications folder and if the file is there, move it to the trash. Then do a clean install using a wired Ethernet connection, not Wi-Fi. Wi-Fi will often cause file corruption due to the very large size of the file. If necessary, temporarily move the computer to directly connect it. If that is not possible, then you may have to do several downloads until you get one that installs successfully. A download should take an hour, more or less and the installation could be the same depending on the speed of your computer. I hope this works for you. Big Sur is great. Enjoy it.

Remember that the installer deletes itself from the Applications folder after running. Make copy (Cmd+C) and paste it to our Public folder (Cmd+V) Give it a new name.

I would also recommend making a bootable USB installer too, in case you need to make a fresh installation. Again, do this while the installer is still around in the Applications folder or copy it back and give it the original name ("Install macOS Big Sur").

I assume you will have an up-to-date Time Machine disk already!

https://support.apple.com/en-us/HT201372

Nope.

You must download it.

Glad to hear you got it done. Enjoy.