how to convert psd to png using api

You could automate that process with a batch action or script but that not an API Application interface its automating Photoshop's processing using feature provide by Photoshop. Scripting and Actions. Adobe is developing an API for web applications so they can use Photoshop via the API.  create api integration 

You can't "convert" as such. The file has to be opened and resaved/encoded into the new format.

Batch actions and the Image Processor Pro can do that.

Take extra care if you need the icc profile embedded. A regular "Save As" will strip it, so you need to use an action with Save For Web, which embeds the profile correctly.

Could you elaborate on what you want to achieve exactly?