Your Sling Wallet always contains USDP, which is a stablecoin pegged to the US Dollar issued by Paxos Trust Company LLC.
For ease of use, the Sling Money app always displays this balance in USDP and a balance in a display currency which you can change in your Settings. You can set this primary currency to almost any currency in the world.
Step by step
To change the currency type displayed at the top of your screen:
Tap your avatar at the top left of your transaction screen
Tap 'Display currency'
Browse the list or type its name on the search bar
And that's it! The new currency will be displayed when you return to the transaction screen.
You can also change your primary currency at any time. For example, if you're travelling in Thailand, you can update Sling Money to display your balance in Thai Baht. Then, when you cross the border into Malaysia, you can update your primary currency to Malaysian Ringitt.