It looks like problem not in network. I have tried update from console. And receive following message:
softwareupdate -i macOS Big Sur-20D74 --verbose
Software Update Tool
Finding available software
Downloading macOS Big Sur
Downloading: 100.00%
Failed to download & prepare update: Error Domain=SUMacControllerError Code=7740 "[SUMacControllerErrorPrepareFailed=7740] Failed to perform Prepare operation: [MobileSoftwareUpdateErrorDomain(MSU):MSU_ERR_FILESYSTEM_DOESNT_VERIFY(3)_1_MobileSoftwareUpdateBOMErrorDomain:1|FullReplacementRequired|]" UserInfo={SUMacControllerErrorIndicationsSummary=|FullReplacementRequired|, SUMacControllerErrorIndicationsMask=2, SUMacControllerErrorIndicationsDescription=-> Full replacement is required [as opposed to incremental patch]., NSUnderlyingError=0x7f81a7806610 {Error Domain=MobileSoftwareUpdateErrorDomain Code=3 "'./System/Library/dyld/dyld_shared_cache_x86_64' did not verify and is not on the exception list." UserInfo={target_update=20D74, NSLocalizedDescription='./System/Library/dyld/dyld_shared_cache_x86_64' did not verify and is not on the exception list., NSUnderlyingError=0x7f81a7806690 {Error Domain=MobileSoftwareUpdateBOMErrorDomain Code=1 "contents of '/System/Volumes/Update/mnt1/System/Library/dyld/dyld_shared_cache_x86_64' don't match bom (29684074 vs f48e5f3e)" UserInfo={NSLocalizedDescription=contents of '/System/Volumes/Update/mnt1/System/Library/dyld/dyld_shared_cache_x86_64' don't match bom (29684074 vs f48e5f3e)}}, MSUFullReplacementRecommened=true}}, NSDebugDescription=[SUMacControllerErrorPrepareFailed=7740] Failed to perform Prepare operation: [MobileSoftwareUpdateErrorDomain(MSU):MSU_ERR_FILESYSTEM_DOESNT_VERIFY(3)_1_MobileSoftwareUpdateBOMErrorDomain:1|FullReplacementRequired|], NSLocalizedDescription=Не удалось подготовить обновление ПО. Повторите попытку.}
