The function's name is "update" since only items in the transaction scope (Order Center) can be updated, but in terms of the "cart" - items that does not yet exist in the cart will be updated, so it's a kind of "upsert" in terms of the cart.
Example:
pepperi.api.transactionScopeItems.update({ transaction:{UUID:"1af91512-81ef-4897-afb1-55d51f0a1948"},objects:[{ item:{UUID: "c1e3fa63-6ca3-4ac2-82bf-c75a43ad138a"}, TSAHello:"hi"},{ item:{UUID:"4a4bfe74-e948-46da-8a61-01be248e42c2"}, TSAHello:"bye"}]});
Returns a response object with the list of updated items as following:
{"success": true,"result": [{"id": "c1e3fa63-6ca3-4ac2-82bf-c75a43ad138a","status": "updated","message": ""},{"id": "4a4bfe74-e948-46da-8a61-01be248e42c2","status": "updated","message": ""}]}