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.
Judith "You have to be odd to be number one." Dr. Seuss