Apple Released macOS Big Sur today with fresh design, Control Center, Safari Privacy Report, messages updates, Maps overhaul, etc. It is FREE to download and installs on pretty much any recent mac products.
Some users — as usual — reported errors and issues installing, hopefully it will be sorted out soon. If you have any older software, or even newer software, check if that software is compatible with macOS Big Sur before updating. Or take a complete backup before upgrading. It is harder to downgrade a macOS, you would end up installing a fresh copy.