Go to the Setting tab and select Manage Wallet. 

Choose the wallet you want to back up → Select Edit 

In the Edit Wallet screen, press Show Backup Phrase.  

You have 3 backup options: 

1) Keystore file 

2) Private Key 

3) Mnemonic seed phrase. 

You only need to use one option, but it is recommended to back up using all three. Save your backup safely and NEVER share it with anyone.

* Note: If you did not import the Mnemonic seed phrase when importing a wallet, you can not exporting the Mnemonic seed phrase.