Finding the "Build Variations" tab is one way to change it, but what if I've accidentally hidden it?

To change the build variant Android Studio uses, do one of the following:

• Select Build > Select Build Variant in the menu.
• Select View > Tool Windows > Build Variants in the menu.
• Click the Build Variants tab on the tool window bar.

Copied from: Change the build variant § Build and run your app  |  Android Studio  |  Android Developers

This is a screenshot of the first approach:

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