Why is there no Software Update option in my Apple Menu?

That is because at the time an UpGrade was offered by Apple - the User chose not to upgrade. Now, the time has passed and is not longer offered through the Apple Store.

To Get an upgrade How to get old versions of macOS. You will require knowing the Age of this computer >> About this Mac << will provide this info. The Age of the computer will Govern what next version of macOS it Qualifies to upGrade to. Verify the requirements of the new OS and then decide which version it can handle. Then use Safari Only to start the download as Other may not work.

Notations: Make a Time Machine Before Attempting the upgrade. Starting in Catalina and above - all support for anything 32 bit is removed and there is no workaround for this.