There is no option to reverse the ordering of your blog posts (reverse-chronological is the default and only option available). However, if you want an older post to appear at the front/top of your blog, you can use the “sticky” status on it:
I have two different wordpress blogs. One shows the last post first and the other is showing the last post last. I see here that you said it is not an option, so why are they different. I want the last post to be first one when opened on both. Did making one post sticky affect that?