If you decided to change the database block size after database creation,The only possiblity is to export the full database and create a blank database with required DB block size and import the full database which was exported earlier.
High level steps are:
- Take a Full export of the database you wish to change the db_block_size
- Delete the database
- Create a fresh database with the required db_block_size
- Import the Full database which was exported.