如何通过javascript更改jwplayer配置

我有一个配置的jwplayer，它有这样的选项：

var playerInstance = jwplayer('player');
playerInstance.setup({
mediaid: "...",
image: "...",
title: "...",
playlist: [{
...
}],
width: "100%",
height: "100%",
aspectratio: "400:157",
});

如何以编程方式更改aspectratio选项？