To keep the bnUSD price stable, you can always retire it for $1 of sICX from the collateral pool, minus a fee. The fee is adjustable via governance, so be prepared for changes if you're running a bot.
To retire bnUSD:
Go to the trade page and click 'Retire Balanced assets'.
Enter the amount to retire, then click 'Retire bnUSD'.
Complete the transaction to receive sICX.
Every retirement reduces the debt for a group of borrowers in exchange for some of their collateral. You can retire up to 0.1% of a group's debt in a single transaction, so the maximum amount varies each time. If the transaction fails, try again later or retire a smaller amount.