How to send APC to explore?

I dont see any option for unused vehicles to 'explore'.

You can recall them. You can use diamonds to purchase a recall. However, it isn't clear HOW to use the item once you have purchased it

It is the construction vehicles that can now 'explore'. From the main screen in your city (not the world map) tap the construction vehicle icon in the top left (tools icon). This will show your vehicles. If they are not currently being used then you will have the option to 'explore'. Tap this and choose the amount of time to send them out for. You can recall them at any time but it will cost.