If you are intending to buy S9 / S9+ at Samsung Experience Store, then you can just bring down the device for evaluation on the spot. They will then deduct the amount that you need to pay for.
Just take note this is not available at Changi Airport Terminal 3 Outlet.
Otherwise, if you have already bought the S9/S9+ then you will need to download the Samsung Trade Up (SG) app for Android devices from Google Play Store or Trade Smartly app for iOS devices from the App Store