When you upgrade a theme from a ZIP file you downloaded, you’ll get a message that looks something like this.
This is WordPress telling us that you already have the theme installed. It’s easy enough to get around this though. Navigate to Appearance > Themes in your WordPress admin. From there, activate a default theme like Twenty Twelve.
Next, you’ll need to delete the old version of what ever theme you’re trying to upgrade. In our case, Vantage.
Don’t worry. All your settings, content, images, etc are safe. Just keep in mind that if you’ve edited the theme files directly then you’ll lose these. That’s why the WordPress community is generally recommending using child themes.
Now, click on the Install Themes tab, then the upload button. From here you can upload the ZIP file. Once that’s done, activate your theme and you’ll be running the latest version.