If you want to alter the end date of your token sale, call the function
updateICODate(uint256 _startTime, uint256 _endTime) from the owner account, which probably is the account you deployed the contract with.
To do so, go to MyEtherWallet (please DON'T ignore any safety warnings from your browser).
At MyEtherWallet, you can enter your contracts address, and its ABI. Then use this website to convert a start and an end date to the unix timestamp, which should be passed to the function.
After the transaction is mined, your ICO should be extended