The English rules for one tile state "The owner of this card can switch one of his already placed contracts with one of the contracts available this round. This ability can only be used once immediately after the owner has chosen a contract."
The iconography does not show a contract, however, but rather only a city card being swapped for a city card. A contract is defined as a set of three cards, one of which is a movement card. Movement cards are no longer "attached" to the city and goods card with which they come in a contract once the contract is taken, however, so it's difficult to see how you could take one of your "already placed contracts" and switch it with an available one, since your "already placed contracts" have no movement card but the available ones do include a movement card.
Perhaps the intent is for it to be used on a turn when you've pushed your luck to the point where you wish you could "go back", and this is a roundabout way of allowing that. That is, it allows you to pick up the contract you don't want (thereby "choosing" it) and "immediately after" putting it back in the queue and swapping it for whatever you skipped-over before. But then why say you can swap any "already placed" contract?
Perhaps they mean the non-movement card portion of the contract? Then why have a picture of just the City card?
FWIW, the German rules also state that it's for the full contract, not just the city card.
Given the iconography and given that there's also a Queenie ability card that lets you swap a goods card for one of two random goods cards, I'm leaning towards the intent being that after taking your turn you can swap a city card in your tableau for a face-up city card you "passed by". And, maybe, for a face-down city card, if you wanna take a chance.