How to remove text label of "Edit Presentation/Show Presentation" button?

I have tried both in mac os x 10.14 and ubuntu 16.04. The "Edit Presentation" and "Show presentation" toolbar buttons of nbpresent are combined with an icon and a text label. They are so long and displayed in a second line of toolbar. So I want to remove the text label of the buttons, but I can't figure it out. Does anyone can help?