You can't because the other nodes will reject it. And there are so many of them... Which is why other altcoins with fewer nodes are more prone to fail. If you have ever heard of the 51% attack, it means you need to have more than half the nodes worldwide for them to validate your fake transaction. Again, Bitcoin having the most nodes than any other, is the strongest in this.

A valid transaction needs to fit, its like trying a key in the wrong keyhole, the fake one won't ever fit. Furthermore, after that key fits, the next one goes connected to yours, which has to fit as well. They all form a chain... the famous blockchain. New transactions go on top of the chain, and all of them behind fit to each other. This also ensures they can never be altered, attempt to change one and the whole thing becomes invalid, and rejected by the nodes. So your transactions remain in there forever, and nobody can alter them afterwards (no erasing, covering tracks, or manipulating of any kind).