If it is star, then the first thing you need to do is find the star point. *runs of to find image*

A quick google gave me the lower of the two images on this page - Star Delta Start Up Principles
(ignore the writing/figures, as it's explaining something you really don't need to worry about!)

as you should be able to see, the lefthand is Star configuration. What you need to do, is split the star point, then reconnect the coils in Delta (right hand image), and apply the power inputs to the delta points.
The motor will still be 0.25kw, however it will now require almost twice as much current, which should be around 1-1.2A of the top of my head. You can use a bigger inverter, however you'll need to alter the current limit settings to ensure it limits the amount of current should the motor get overloaded for any reason.