Very simple. While writing blog in blogger editor just hit code view option at the right upper corner of ur editor. Write <h1> or <h2> tag before the headline and at the end of the headline close the tag </h1> or </h2>.
but if i edit my previous blog posts with h1,h2,.. will that affect my next search engine traffic? thanks again
Possibly. Depend's how poorly they are configured at the moment... Google is caring less and less about code. It's what the end user sees that counts... with some limitations of course...
In blogspot, by default blog title is the H1. So, if you write manually h1 then their will be 2 h1 tags. So better to keep the same and write only h2, h3 manually.