Never heard about Note 20 Ultra failing to update software. But I can help you out. You have 2 options:
1. Update using Samsung Smart Switch.
2. Visit a service center and ask them to flash the latest firmware. (Be warned that this process will erase all your data, so take a backup before you proceed)
If you have any issues or anything to know, hit me up and I will try my best to help you.
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