If you want to change a text that is in already in the button goto page 1 -> current language -> button on the right.

No, I want to place new button in some pages

You usally have the option to use a picture as button or backround.

So you need to create it, or download a picture of a button and create/edit the text in it with a graphic program.

If you want your button to appare into all the pages you can create it into the main menù as a new level that you set up as link-to...

Else, if you want to put your button only into some pages, you can create it as a html object by using the apposite widget (only present in version 9); or you can proceed as Andre E told you, by including a link into a graphic content.