1) Go to the Mint aUSD tab. Go to the Vault you'd like to close and click Manage.
2) Click Pay Back.
3) Enter the amount of aUSD you'd like to pay back. Note that you have to pay back the full minted amount plus the accrued Stability Fees. Click Confirm.
4) Click Withdraw.
5) Enter the amount of collateral that corresponds to the amount of collateral you deposited. Click Confirm.