hi not able to down load the latest software for my mac updation

This computer does not meet the requirements for the newest version of macOS. This is why you aren't able to download the newest version. When you go to update the software, your computer is going to download the highest available that you can run with your hardware. If it is telling you that your mac is up to date, then it is already running the highest available version that hardware supports. If you want to run the newest version of macOS, you are going to have to upgrade by getting a newer machine that does meet the requirements.

What model/year is your Mac?

Here are the requirements for Big Sur: