If you want to force settle your bitBTC, go to your account overview and click on "bitBTC" in Balances table, Assets column. In the expended list you will be given a choice to force settle your asset. If you click it, 24 hours later your bitBTC will be sold to the least collateralized borrower at whatever feed price will be at that moment, and his position will be updated. But don't do this, bitBTC:BTS has pretty good liquidity and you'll probably get better price at the market.