Text Tool ⇧T
Choose the Text Tool. The cursor becomes the I-beam whenever it is located in a place that can accept text. I only find it useful in the Score Editor. In the main window only useful if you have markers that you want to rename, which can be done with a double-click on the marker. I typically don’t rename my regions a lot.
Use the Text tool to rename regions and other items, or add text to a score in the Score Editor.