You have two ways to cash out from Atomic Wallet.
1. Click on the "Send' button.
2. Insert Amount, Address and your Atomic Wallet password, and Destination Tag (if required). If you send your XMR to another Atomic Wallet XMR address, you can leave Destination Tag field blank.
3. Calculate your Amount with fee.
For example, if I have 1 BTC and want to transfer it to other address. The fees will be 0.0002. It means that i will be able to send only 0.9998 BTC.
If your transaction is not going through, try to decrease your amount. The Node can estimate your transaction as a Dust amount. (Which means that you're trying to send sufficiently small funds that makes it almost useless transaction regarding network fees).
4. Click "Send" and check your transcation's status in the blockchain.
In the "History" tab click on the transaction and then on hash.
Export Private Key function.
You can cash out by exporting your private keys. By this operation you will insert your private keys in other services/wallets.
1. To export your private keys go to "Settings" -> "Private keys" and Insert your password.
2. You will see the list of private keys. To copy your private key -> just click on it.
3. Then paste it in any other wallet that support the coin that you're want to export.
If you want to export Monero (XMR) pricate key you will need to deal with Private View and Private Spend keys:
First line is a Private View key.
Second line is a Private Spend key.