I m not getting any such option in bitcoin-core Ui and documentation http://bitcoindaily.org/bitcoin-guides/bitcoin-core-tutorial/
Bitcoin Core (since 0.13) uses HD after BIP32 with only hardened private key derivation. Exporting the xpub would be useless, because other apps could not derive public keys (hardened derivation requires the xpriv).
Though, you can export the xpriv with
dumpwallet <filename> (RPC or Debug Console). But be careful with that.