使用 require
// 安装包
composer require elasticsearch/elasticsearch:~6.0
// 已安装 升级
composer require elasticsearch/elasticsearch:~6.0
// 已安装 降级
composer require elasticsearch/elasticsearch:~5.0  
// 忽略版本  
composer require elasticsearch/elasticsearch:~5.0 --ignore-platform-reqs
// 删除依赖包 
composer remove elasticsearch/elasticsearch --ignore-platform-reqs

使用 update
vim composer.json
"require": {
    "elasticsearch/elasticsearch": "~6.0"
},

:wq

#升级到6.0
composer update elasticsearch/elasticsearch

vim composer.json
"require": {
    "elasticsearch/elasticsearch": "~5.0"
},

:wq

#降级到5.0
composer update elasticsearch/elasticsearch