by default it saves as .psd how do i change it to .png by default because its buggin to change each and every time !!
The PNG format can not hold the minimum information that is contained in an image in Photoshop, even if it's a single layer RGB image. For example, the PNG format doesn't support color profiles, and an image in Photoshop always has a color profile.
So, saving as PNG is really exporting. When you load the image into Photoshop again, there is no guarantee that all settings are the same.
It will always save as a PNG... as long as you haven't added anything that a PNG can't contain.
Basically it sounds like you're adding layers. As soon as you do, Photoshop will save your PNG file as a PSD to save your layers for you. To stop this FLATTEN the image and then press CTRL-S. It will save the image to whatever format you opened it in.