it always says 500 internal server error whenever i try to download macOS BigSur im on macOSX Yosemite

https://discussions.apple.com/thread/252893307

The Age of this computer will Govern what newer version of macOS it will Qualify to install. Use About this Mac from the Apple Icon on Desktop - the required Information will be there.

With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install. Make sure to use Only Safari to commence the download as Other may not work

Make a Time Machine Backup Before attempting. Notation - Mojave is the Last version to support 32 bit applications / extensions and Drivers. Catalina and Big Sur Only support 64 bit and there is no workaround.

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.

In your picture above - using Chrome browser from the link to download Big Sur will not work. Use Only Safari

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, 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

Please - by what method is Big Sur being attempted to download. As much detail as possible, Year of Computer, Empty Space on the internal drive, type of Internal Connection Wifi or Ethernet cable to Router, Speed of connection from your ISP, are you running behind a VPN, AntiVirus software installed

Maybe parsing words " won't update " implies it was Successfully downloaded but does not install ??