how to update my OS to Big sur?
My new software requires OS Big Sur. I can't install it.
My Mac is late 2012
Is it because that Apple does not support it?
iMac 27″, macOS 10.15
A late 2012 iMac can be upgraded to Catalina 10.15.7 but no later.
See the following from Identify your iMac model - Apple Support
iMac (27-inch, Late 2012)
Model Identifier: iMac13,2
Part Number: MD095xx/A, MD096xx/A
Latest compatible operating system: macOS Catalina 10.15.7