Ways to edit metadata of a PNG file?

From the Wikipedia page for 'pngcrush',

The main purpose of pngcrush is to reduce the size of the PNG IDAT datastream by trying various combinations of compression methods and delta filters. However, pngcrush can also be used for various manipulations of PNG images, such as changing the bit depth, removing unwanted ancillary chunks, or adding certain chunks including gAMA, tRNS, iCCP, and textual chunks.

For the MAC OSX, there is a freeware called 'PNGCommentator'.
Have not seen it on other platforms.

PNG metadata can now be added and edited at the click of a mouse. Version 1.2 adds support for batch processing and customisable presets, allowing for much faster workflows. Text macros extend this even further, allowing the software to automatically insert data such as the date or filename.

MetadataTouch can edit PNG metadata, but it is not free.

If metadata is added to PNG files, such as keywords, it doesn't "stick" permanently. I found the only safe way was to use Bridge > Photoshop > Image processor and batch change the PNG to a JPEG, PSD, or TIFF file. I used tiff LZW to save space and even then it is a larger file than the png. I needed this to be able to add keywords and descriptions to pngs in scrapbook kits.

This was not my idea completely. I was changing the file extension in a more round about way and found this presented by a scrapbooker whose name and site I forget. I think her first name was Kayla.

TweakPNG is a free software that can edit PNG metadata (Windows only).

ExifTool can edit PNG metadata. You can find the list of the supported tags here.

Exiftool is a free command line tool, but there are GUI tools (listed on the ExifTool website) using the ExifTool engine.

The new-ish default Windows 10 photo viewer ("Photos") lets you edit PNG metadata now:

• "..." menu (top right)
• "File info"
• Can change the name or Date Taken fields there

If you only care about date and not time, you can change other date properties directly in Explorer (via preview panel) or from the right-click Properties window