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 Theme (blue button) again
Then you need to click 'small arrow ⬇ ' beside theme name on top
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 Sales Channels > Online Store > Themes > Customize Theme, then in the upper right, you should see a link called "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 "Info" link the upper right, 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 "2.2".
Once you find your theme version, you can find out how to upgrade your theme by following this link here.