AVIF format is unfortunately not supported by default Microsoft Paint, which makes many users struggle with using .avif images and they would prefer to have the pictures (typically downloaded) in one of the standard formats, such as PNG.
Your best option in this case is to use one of the graphics tool that supports AVIF format, for example Paint.NET or GIMP. These can be used to resave .avif files to PNG, pretty much achieving avif to png conversion in the process.