If a bitcoin transaction gets a confirmation, there is no turning back from there.
It's not 100% true.
A miner (attacker) with a very big hash power may be able to reverse transactions with 1 confirmation.
That's why exchanges usually wait for 6 confirmations.
Assume that you have received a transaction that has 1 confirmation. There's a possibility that another miner has mined an alternative block which has now 1 confirmation. Now we have two chains. The alternative chain may receive next confirmations and becomes the main chain.