When it comes to features of any specific theme it is usually best to ask the theme developer directly.
=> To edit posts go to Posts > All Posts. Click on the Title or the hover/edit link to edit post content.
=> If by "defined posts" you mean the featured posts that display on the home page that is generally the 3 most recent post by default. Or, in some themes you can use the Featured Posts widget in that homepage space.
If this is the case, you can create a category -- such as Featured. Then, "file" the posts that you want to display on the home page in that category. Then use the Featured Posts widget.
Appearance > Widgets > Featured Posts: move that to the widgetized area on the right for that space on your homepage and choose your settings including selecting the Featured category.