This is a YADQ (Yet Another Doc Question), and consequently in violation of the etiquette for this forum.
If you would have read the documentation on db_block_size, you would have known you can not change it, without recreating the database.
The 'steps' to accomplish this are obvious for those who can be bothered to read, so there is no need to repeat them here.
1.First take a full database export of the existing database (addition to this take a complete cold backup for replication purpose).
2.Drop the existing database.
3.Create a new database with the same settings (create the non-system tablespaces , all datafiles should be in same location as in dropped database ) except db_block_size.
4.Finally Import the exported data into the newly created database.
If it fails then go back to your old database using available cold backup..