Here's a little tutorial!
When you are working on your manikin, there is a tool on the right called «Posture Editor». Use this feature and click on the manikin you'd like to modify. A pop up will appear with all the adjustments you need.
You have many choices to modify your manikin from there, but my favourite one is to click on the desired part to modify, and then use the sliding bar to move it.
There are many parameter, experiment with it and you'll have what you are looking for in a few minutes!