If the first transaction was sent with RBF enabled, then you can replace it with a higher fee transaction.
So, what should i change/set in a tx for it to be marked as RBF enabled?
There is surprisingly little i can google about this.
You cannot change it after the fact. Since the transaction has already been broadcast, you cannot change it to be RBF enabled. That can only be done when creating the transaction, before it is signed and broadcast. Your transaction is not RBF enabled.