The attack on the Blockchain (Bitcoin) is a very difficult task to successfully accomplish in terms of processing power.
As a result, it is almost impossible to change a Bitcoin transaction that has already been signed. Obviously it is possible to change the status of a transaction through a technique known as "Double Spend Attack", however it requires enormous processing power.
In theory, the same Bitcoins are spent twice.
The attacker should have a total network computing power must be greater than 50%, however the value of Bitcoin would be reset to zero (people would sell).

ATTACK
How does the attack happen? Without going into too much detail an alternative chain (Blockchain Fork) is created with an invalid transaction.
However, if this "parallel" transaction (with the same amount in Bitcoin) is longer than the lawful one, all the nodes in the network will consider it lawful.
The "real" transaction will be considered corrupt.
Basically, person A who uses this technique seeks a service from person B.
Person A will create two Bitcoin transactions: one that includes payments for the product he is looking for from B, instead with the other ... he pays the same amount to himself.
We will have the transaction from "A to B" and then secretly start mining the block which includes the transaction from "A to A" (towards himself).
The transaction will be placed first in one block and then in another.
Every time a transaction ends in a block, it receives 1 confirmation (5 or 6 are needed).
B will give the service to A and if the operation is confirmed it will send the agreed product / service.
In reality the illegal transaction (towards itself) will have a longer chain of blocks so it will be considered lawful, the real one instead ... will not be confirmed.
The same BTC will be spent twice (in theory).