well, the easiest way would be to add zeros. you would alter the code so 1BTC will be worth 1000BTC. nothing actualy changed (since you could also cut 3 zeros after the comma)
This is more difficult in practice than in theory. If Bitcoin were a centralised organisation like a company then the CEO would announce the stock split and everyone would have to follow.
But
unfortunately, Bitcoin is decentralised, and no one has the authority to announce this change.
We have to find a solution that allows for a gradual, organical transition (as opposed to a coordinated switchover).