Go to Colors > Posterize
It will reduce the amount of available colors in the image. The lower the number of colors the lower the file size. This also lowers the quality
Most of the time you can lower a png pretty low and itll still retain quality. Usually 72 is a good number to start at. If its not small enough then, keep goin lower

Also be sure to autocrop the image, extra transparency will cause a lot of extra size.
If you still cant get it low enough, you will have to remove all transparency and lower the physical dementions