How to modify button label

Attached is a screenshot of a form (italian language sorry).
I have a couple of fields (auto-complete text) which represents colors and materials. I create a linking to the select list , in order to insert color and materials when they are missing from the list.
Is it possible to modify the label of the button ?
I think I did not find the corresponding button in the toolbar section , and I do not know where to search for it.

Thank you very much

[ATTACH=CONFIG]n65760[/ATTACH]

screen1.jpg