The backup key (or manual key) is only shown ONCE whrn you enable the 2fa. Now, you have to FIRST disable the 2fa on the exchange (be careful, 24h withdrawal suspension on binance when disabling). Then delete the 2fa in GA app. Re-enable, keeping an eye out for any code that appears on the page where you scan the barcode. Take that code and put it on a piece of paper. Store that piece of paper in a safe location.