可以随便拥有,可以随意做(尤指那些不被别人看重的东西或事)If someone is welcome to someone or something, they can have it or do it, especially because it is not wanted by anyone else.
You're welcome to it - I can never get it to work right. 你尽管用——我就从没能让它正常运转过。
If they want to change the rules, they are welcome to try. 如果他们想改变规则,尽可以试试。