For more information on Rive Text see the respective runtime and editor documentation.

Text

Runtime docs on Rive Text.

Text Overview

Editor docs on Rive Text.

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Update Rive Text in Unity

A unique run name must be set in the editor to be easily discoverable at runtime. See the text runtime docs for more information.
A text run can be updated from an artboard instance by providing the name and new value: 
Artboard artboard;

....

artboard.SetTextRun("textRunName", "newValue");
Note: This API only updates text runs on the given artboard, and will not update text runs on component instances.

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Nested Text Runs

For more information about updating nested text runs in Unity, take a look at this example.