怎么更新composer版本？

我在命令行输入composer self-update，报错了，以下是报错信息：

[Composer\Downloader\TransportException]
The "https://getcomposer.org/version" file could not be downloaded: SSL operation failed with code 1. OpenSSL Error messages:
error:14090086:SSL routines:SSL3_GET_SERVER_CERTIFICATE:certificate verify
failed
Failed to enable crypto
failed to open stream: operation failed

我该怎么解决呢？