From the Bionic Cat menu, choose "install software"
From the window that pops up, choose "Install" at the top and find the option to "unblock Big Sur."
Click "install" by "Unblock Big Sur." It may remain at "installing," but that's normal.
Head to the Apple Menu >> System Preferences >> Software Update. Start the Update to Big Sur. Alternatively, head to the App Store to find and download Big Sur.
Once you have downloaded the Big Sur installer, return to the "Unblock Big Sur" app and install it once more before beginning the install of Big Sur.
Shoot us a note at support@bionic-cat.com if you have any troubles.