how to change the png file background to transparent

You can actually do that using Paintbrush (Mac)

Select a foreground color - any color will do as our intention is to remove the background.

Next, drag the opacity slider from 100% to 0%. You should see your selected color turn into a grayish tone.

Now click the fill icon and click anywhere in the background.

You should see the background is now transparent.

No installation needed - use Preview.

1. Open the image in preview
2. Click the "toolcase" icon in the toolbar
3. Select the "magic wand" tool
4. Click and drag to set the "radius" until it selects what you want to get rid of
5. Press Backspace
6. Rinse and repeat for other areas as you would with fill

This is a late answer, but this is much simpler than any of the other answers. For the googlers!