Information in the blockchain cannot be deleted or replaced, by design. But it's possible to put in new information that supercedes the previous information. For example if you're using streams, you can publish multiple items under the same key, then when reading only retrieve the most recent item for that key.
As I write this we're working on functionality to make it easy to embed hashes only in a blockchain, and deliver payloads over the network. This will make it easier to remove old items from storage.
