It is so easy to use Bither app.

Generating Coinbase bitcoin receiving Address

first of all you need to check your Coinbase bitcoin receiving wallet address which is shown by a QR-code (you can find out how, from here).

Sending fund

Then you should try scanning that QR-code by your Bither app. for doing it, you need to choose one of your wallet (from the middle menu which is indicated by a big B and showing total amount of your funds) then you should choose the preferred wallet. then use the send icon and scan QR code and type the amount go ahead and send funds.

caution

by the way, you should be aware of your transaction fees. Bither does not propose any offer automatically; so you should choose your own policy for transaction fee by monitoring mempool to get informed use this website to see which tx takes how long to get confirmed.