Unfortunately, there isn’t an easy way to update it, and most likely you wouldn’t want to update it anyway.
You see, although the admin panel is reporting that the theme you’re using is “Sage Starter Theme” and we are in fact the creators of Sage, we didn’t actually build the theme you’re using in its current state. As the name implies, Sage is just a starter theme.
This is what the full theme looks like out of the box: https://roots-example-project.com/hello-world/
If that looks like an incomplete theme, it’s because it is. That’s deliberate. We made Sage so that fellow developers could download it and use it as a starting point for developing their own themes. We provide tools to assist developers with creating themes, and you can think of “Sage” as the brand name that we’ve given to our toolbox.
It sounds like your developer used Sage to build out a theme for you, but he or she didn’t rename it from “Sage” to “Ed’s Awesome Theme” or some such, which is why you’re here.
I would recommend reaching out to the developer of the theme if possible. Otherwise you should consider hiring another developer to review the theme to make sure there aren’t any vulnerabilities in it.
If you want someone from the Roots team to take a look at your theme, you can schedule a call with us via this page: https://roots.io/services/