Using the Alignment tab
Open the Text Box Properties dialog box.
Then, do these steps to choose the vertical alignment of the contents of the text box:
Click the Alignment tab.
Click one of these Vertical Alignment options:
(top
aligned), 
(center aligned) or 
(bottom aligned).
Click outside of the dialog box to close it.
Click the end of the Change Layout control to hide the controls.
Note
The text box can contain one, two or three languages. The alignment is applied to the entire text box, not to individual languages in the text box.
Unlike setting a style, this property is for the current text box only. It will persist if you use Duplicate Page. However, if you change it later in one of the pages, it will not change in any other pages.
The vertical alignment you choose do not affect the alignment in ePUB files.