I'd like to display some of the custom fields in a different position (sidebar), how do I do that?

Hi Feng,
When you say Side Bar do you mean in a Joomla Module position ?

Yes. say in joomla 3, the protostar template. I'd like to move it to position 7.

I understand you could create an override for item.php, but how do i do it within the actual template?

I don't think you add it to a template overide, if your using K2 v2.6.2 I think you can use either the K2 Content or or K2Tools Module but I think K2 Content would be easier because you can make a custom overide of the module then customise the layout to display a specific extra field using this code .