How to change text in template using event?

Question

Yii

There is an application source code that is highly undesirable to touch.
I need, using a new module, using an event to change the text inside the templates. For example, throughout the site there is the phrase "Old text" that I need to replace with "New text".
I have created a new module. Presumably, using the EVENT_BEFORE_RENDER event handler, replace the template.
I assume that I need to use the event to pick up the moment when the template is rendered, and then from my module write the function to replace the string I need.
I still can't figure out how to write events. I found a function in the controller that renders one of the templates I need, but I can’t figure out what to do next.
Thank you in advance!