Question: Can I change transaction that has been already registered in a block ?
Answer: Basically no. In most of the Blockchains, like Ethereum or Btcoin, transactions are immutable, meaning that if they are registered successfully into a block they can not be modified or deleted again. There might be way to create an inverse or a modifying transaction as well, however it means that the database will store both the original and the inverse transaction and the rest is only interpretation. There might be on a long run under the Hyperledger foundation project some solutions that might have a pluggable storage mechanism making eventually possible to explicitly delete a transaction, however such a special algorithms will be probably not regarded as Blockchain, rather as general distributed ledger solutions.