Q: When attempting to update my Mac to a newer software version such as Maverick 10.9, I get a message that indicates there are no updates available.  What am I missing?

You are not telling what your hardware is. Open the Apple menu whilst holding the 'alt' key on the keyboard (it may be called 'option' on older keyboards). Select the System report or 'Apple system profile'. That will say what the model name and model identifier is, please post that info here.

You can also look at the specifications for each OS, I suspect you could have a Mac that cannot install these newer OS's.

If it can install a newer OS you must use the App Store to purchase them open that & see if you can purchase a copy of a newer OS - it will only allow you to download compatible OS's on a compatible Mac.

System requirements for OS X Lion and Mac OS X v10.6 - Apple Support

OS X Mountain Lion system requirements - Apple Support

OS X Mavericks system requirements - Apple Support

If you mean you're using Software Update, you won't find it there.

It is a bit confusing that Apple insists on holding onto the OS X moniker when it keeps releasing new major OS upgrades with the same basic name of "OS X". For pretty much any other software I've ever seen or used, a point release is an update, not an upgrade. But for OS X, Apple is using the first point number to signify major OS upgrades.

Snow Leopard, 10.6.x is a major release. 10.7.x is another, and so on. No update will be offered for major upgrades through Software Update. You have to open the App Store in the Applications folder and click on Yosemite there. If you Mac supports it, you'll be allowed to purchase, download and install it. Yosemite is free, it's just that anything you get from the App Store is designated as a "purchase" under your App Store account.

You cannot get Mavericks. For reasons Apple won't divulge, they pulled it when Yosemite, 10.10.x was released. You can purchase Lion 10.7.x or Mountain Lion, 10.8.x if you don't want Yosemite.