My current version is macOS 10.15.6.I want to update to macOS Big Sur 11.5.1 . But software update is shown for macOS 10.15.7.How can I get Big Sur?

Open your Safari browser and click on the link below.

How to upgrade to macOS Big Sur - Apple Support

Check your mac can upgrade to macOS Big Sur,

MacBook introduced in 2015 or later

MacBook Air introduced in 2013 or later

MacBook Pro introduced in late 2013 or later

Mac mini introduced in 2014 or later

iMac introduced in 2014 or later

iMac Pro

Mac Pro introduced in 2013 or later.

Your mac should also have a minimum of 35.5GBs of free space.

macOS Big Sur - Technical Specifications

General Requirements

OS X 10.9 or later

4GB of memory

35.5GB available storage on macOS Sierra or later*

Some features require an Apple ID; terms apply.

Some features require a compatible internet service provider; fees may apply.

Mac Hardware Requirements

For details about your Mac model, click the Apple icon at the top left of your screen and choose About This Mac. These Mac models are compatible with macOS Big Sur:

MacBook (2015 or later)

MacBook Air (2013 or later)

MacBook Pro (Late 2013 or later)

Mac mini (2014 or later)

iMac (2014 or later)

iMac Pro (2017 or later)

Mac Pro (2013 or later)

Learn about compatibility and how to identify your specific Mac model

Don't use FireFox or WaterFox for this link. it'll mess it up...

https://apps.apple.com/us/app/macos-big-sur/id1526878132?ls=1&mt=12

And caution on 10.15 or higher, to see which Apps will no longer work in 10.15 get Go64...

https://www.stclairsoft.com/Go64/

I have an iMac 2017, plenty of free space, and macOS Catalina 10.15.7 currently. Big Sur used to be available in Software Update, but it isn't now. The App Store download of Big Sur gives the error message that "the requested version of macOS is not available." Is it possible to update to another version of Big Sur?