why can't I update to Big Sur? It's not there when I ask for updates

How to Upgrade  macOS:

The macOS version you can upgrade to depends on the model and year of your Mac.

Click the  (Apple) menu at the top left of your screen and choose About This Mac. 

❗️ – IMPORTANT – Backup your data, now, before you upgrade.

Learn How to Back up your Mac with Time Machine:

https://support.apple.com/en-us/HT201250

❗️ – IMPORTANT – Your 32-bit apps won’t work in Catalina or Big Sur.

https://support.apple.com/en-us/HT208436

Use this free utility to check the compatibility of your apps:

St. Claire Software – Go64: https://www.stclairsoft.com/Go64/

🖥 – Check compatibility of your Mac with newer versions of macOS.

https://support.apple.com/en-us/HT201475

These Mac models are compatible with macOS 11 Big Sur :

• MacBook (2015 or newer)

• MacBook Air (2013 or newer)

• MacBook Pro (Late 2013 or newer)

• Mac mini (2014 or newer)

• iMac (2014 or newer)

• iMac Pro (2017 or newer)

• Mac Pro (2013 or newer)

✅ - Using Safari only, download the macOS upgrade installer app:

Click > Big Sur.