In order to withdraw and sell the LBC coins you will need an account on Bittrex, which is a crypto exchange that lets you buy and sell crypto.
Click on your balance at the top right corner of your account on Libry, you will then get the options to Buy / Receive / Send,
Click on "Send" and enter the amount of LBC to withdraw and your LBC address on your Bittrex account,
You can then exchange the coins for BTC / ETH / USDT
Easiest way to cash out to fiat is probably to exchange LBC to either BTC or ETH and transfer to Coinbase account, which is another exchange where you can exchange the BTC / ETH for fiat, and then deposit the cash straight into your bank account,