You may have been reading another article referring to theme versions, or asked by the support team which theme version you are using and thought "Which version am I using? How do I find out?" This article will help you answer those questions.
For newer theme versions (with Sections) you can find out which theme version you are using by going:
Online Store > Themes > Customize
Then you need to click 'Theme Settings' and your version will be at the bottom:
To download and for more information on the Theme Updater app see here.
For older theme versions (pre-Sections)
To find out which version you are using, go to Online Store > Themes > Customize > Scroll down and click Theme Actions > Theme Info:
Click on that and it'll open up a window with your theme name and version number right after. 
In this screenshot, you'll see the name of the theme is "Responsive" and the version is "5.0".
You may look at your theme settings and notice that you don't have the "Theme Info" link under 'Theme Actions', but don't worry, you can still find your theme version, under the Welcome tab.
Once you click on Welcome, it'll expand to show you your theme name and version. 
You'll notice in this example, the theme name is "Retina" and the theme version is "2.0".
Once you find your theme version, you can find out how to upgrade your theme by following this link here.