I can not update Big Sur in my iMac

An update to Big Sur would be going from Big Sur 11.7 to Big Sur 11.7.6 The most current version

Is that what the User meant by update ?

Moving from previous version of macOS, say macOS 10.14 Mojave to Big Sur macOS 11 is a upGrade

Predicated on the second of the two scenarios and wanting to upGrade to Big Sur.

If this is not you computer situation and updating is really what is meant, please post back and will attempting to help with that issue

Please refer to the suggestion below to possible achieve what the user meant to say - upGrade

The Age of this computer will Govern what newer version of macOS it will Qualify to install. Use About this Mac from the Apple Icon on Desktop - the required Information will be there.

With the above information in-hand - follow this How to get old versions of macOS and verify what version this computer Qualifies to install.  For Best results use Safari to commence the download as Others may not work.

Safari will open a redirect and open the Apple Store where the download can commence.

Does this computer appear as Listed and qualifying for an upGrade to a newer version of macOS? 

The below links will help identify which version of macOS  this computer may Qualify to run.

macOS Big Sur 11 hardware requirements

macOS Catalina 10.15 hardware requirements

macOS Mojave 10.14 hardware requirements

Make Time Machine Backup  is very useful 

Notation - Mojave is the Last version to support 32 bit applications / extensions and Drivers. Catalina, Big Sur, Monterey, Ventura supports only 64 bit and there is no workaround.

Suggest using Go64 to find 32 bit application before attempting the upGrade. Some or many of the Application already installed may require an update or upGrades to 64 Bit Version