Can I edit/customize a skin that I bought in Minecraft? If so how?

If you bought it then you're using either console/mobile version or the bedrock version which means you can't edit them.

If you had the Java version, you could use one of the many sites to create a skin or edit existing ones.

If purchased for the Bedrock Edition of Minecraft (XBox One, Windows 10, Switch or mobile), you can't edit skins & skin packs that you've purchased.

The Java Edition allows you to create your own, so there's no marketplace for purchasable skins.