To have the Bible books in Greek you will need to do one of the options below:
1. Change the book names to Greek in the books table. To do this go to the advanced tab of the Preachit admin and click on the Bible books link. Open each record and change the book name. Keep the english name as is.
2. Switch on the multilanguage setting in Preachit. This option is available in the Preachit component options - click on the options button at the top right of a Preachit admin page - and under the general tab.
Preachit is designed to give maximum editing of the details so you can change most things quite easily. If you are not planning a multi language site option 1 is probably the best.