Sure you can. It depends on how you get your source ePUB (I am afraid a Calibre conversion of a LibreOffice document). If you already have a stylesheet, you should check in the stylesheet if there is an h1 selector. If there is, you can adapt this. If not, you need to add something like:
Code:
h1 {font-size: 1.5em;}
to the stylesheet.
Changing font is more difficult and can give some headaches. It would be better if you have some more experience in HTML/CSS before going there.