1). For this, check each and every folder of Magento if it has proper permission and then try again. For Proper permission you can run Magento Cleanup file and then try again.
2). Go to downloader/lib/Mage/Archive/Helper/File/Gz.php in your Magento directory and replace
$this->_fileHandler = @gzopen($this->_filePath, $mode);
with
$this->_fileHandler = @gzopen64($this->_filePath, $mode);