When using a clip with text on screen you are able to change what text is being shown.
Drag your clip to the timeline and click it to go into the edit window.
Below the preview and mini-timeline you'll see a text input field where you can add your own text. The number of text input field depends on the specific clips and matches the text shown on screen. Above the text input field you'll see a short description of what it is in reference too that is especially helpful for clips with several inputs.
To change the text on screen simply start typing. On the right side you'll see the max character count that starts counting up when you're typing.
Your custom text on screen is not a live preview update and will be added once the project is rendered.
Ie you'll not be able to see your own custom text on screen on the specific clip or when previewing the entire project. This change happens when the project goes through its final export process.