Editing a block is much like creating a new block except all of the fields are pre-populated with the data that was entered when creating this block. If at anytime you wish to cancel what you are doing just click the Back to Main link at the top of the page.
Now you can modify the content in the block and change its allow view options. The only thing you can't change here now is the theme variable you chose when creating a block. This has to now be modified by the layout module's box move.
When you are done editing the block click the save block button and the block will be saved to the database and return you to the list of blocks.
Make sure the changes you made are correct because they will go live as soon as you save the block. Unless you deactivate it before you begin to edit.