To keep the bnUSD price stable, you can always redeem it for $1 of ICX (in the form of sICX). This reduces the total debt in exchange for ICX collateral, so it's best suited for traders without a bnUSD loan.
To redeem bnUSD for $1 of ICX:
Go to the trade page and click 'Retire Balanced assets'.
Enter the amount to retire, then click 'Retire bnUSD'.
Complete the transaction and receive sICX, which you can swap for ICX or unstake from your wallet.
If you have a loan, learn how the retirement process affects your position.