How to use different asset on Futures Testnet?

Hi, I have some USDT on futures testnet. How can I change them into BUSD?

You can trade it from BTCUSDT pair then use the BTC to trade in BTCBUSD pair. Take note that the liquidity may be low for BTCBUSD pair, testnet is not testing your trading strategy. Instead, you should test it on the historical data which can be found here: https://data.binance.vision/

Looks like Multi-Assets Mode is also a solution