You can change the default HTML of Velt Components by providing your own HTML as a child of the component.

<VeltWireframe>
    <VeltCommentDialogWireframe.Composer.ActionButton type="attachments">
        <div className="my-custom-class">
            <MyIcon />
            Attachments
        </div>
    </VeltCommentDialogWireframe.Composer.ActionButton>
</VeltWireframe>