Press on Import / Export tab then press export unencrypted choose base base58 or 64(can't remember which one) format. After you see a word priv and numbers after it is your private key. Copy paste those private keys that has bitcoins inside and import them to either bitcoin-core client using importprivkey command or either import them to a lightweight client such as Electrum or Multibit.
Links
https://bitcoin.org/en/download
https://electrum.org/download.html
https://multibit.org/