You can adjust this by going to the Python Integrated Tools settings.

On Windows/Linux

File -> Settings -> Tools -> Python Integrated Tools 

On OS X

PyCharm -> Preferences -> Tools -> Python Integrated Tools

Then there is a Docstrings section. Change the drop down format to NumPy and press "OK"

At the time of this writing, it is not possible. Several tickets have been submitted over the past several years requesting this feature, presumably it is being worked on.

https://youtrack.jetbrains.com/issue/PY-33833

https://youtrack.jetbrains.com/issue/PY-20243

In the short run, if you need to convert previous projects from the default, you can find the setting is stored in the ".idea/MYPROJECT.iml" file:

  <component name="PyDocumentationSettings">
    <option name="myDocStringFormat" value="Google" />
  </component>

By default, there is no PyDocumentationSettings component, so you'll need to add it. In Linux, you could run the following (very hackish) code in terminal to change all projects in any subdirectory:

new_docstring='  <component name="PyDocumentationSettings">\n    <option name="myDocStringFormat" value="Google" />\n  </component>\n</module>'
find . -type f -name '*.iml' -print0 | xargs -0 sed -i "s|</module>|$new_docstring|g"

If you simply wanted to go from e.g. Numpy to Google, you might just use:

find . -type f -name '*.iml' -print0 | xargs -0 sed -i "s|Numpy|Google|g"

Those are the Sphinx style doc strings.

Python has some strong opinions about doc strings, see PEP257.

Here is a full description of all of the doc string types in Python: What is the standard Python docstring format?

I believe there should be an empty line before first :param to separate description. It fixed formatting for me.

With PyCharm 5.0 we finally got to select Google and NumPy Style Python Docstrings templates.

It is also mentioned in the whatsnew section for PyCharm 5.0.

How to change the Docstring Format:

File --> Settings --> Tools --> Python Integrated Tools

There you can choose from the available Docstrings formats:

Plain, Epytext, reStructuredText, NumPy, Google

As pointed out by jstol: for Mac users, this is under

PyCharm -> Preferences -> Tools -> Python Integrated Tools.