I’m glad it worked.
I suggest you to read through the repeater velo api
The first problem was that your onClick() button event was inside the repeater onItemReady().
This repeater function is triggered only when a new item is added in the repeater.
In my solution there is only the button onClick() event waiting. As the button is part of the repeater we want to know which one triggered the event. Using the event.context we get the item of the repeater that fired the event, then we can read the label of the button inside that given item to update your text.
Hope it helps 