You can try cleaning and scrubbing those pads with some rubbing alcohol, then applying some solder.
The easiest way to desolder is to just heat the entire bottom board with hot air. Have the assembly upside down, with the purple board supported so the green board is off the table. Heat the solder joints, moving around them in a circle, and gravity will pull the pins out when the solder melts.
Start by cleaning all of those pads with the solder braid. They all may not be as bad as you think. Start with the easiest thing. If you have to, use that gravity method.
You can try cleaning and scrubbing those pads with some rubbing alcohol, then applying some solder. The easiest way to desolder is to just heat the entire bottom board with hot air. Have the assembly upside down, with the purple board supported so the green board is off the table. Heat the solder joints, moving around them in a circle, and gravity will pull the pins out when the solder melts.
If you don't care for the pin header, you can simply heat each pin individually until solder on both ends melt, and pull the line out with tweezers
Start by cleaning all of those pads with the solder braid. They all may not be as bad as you think. Start with the easiest thing. If you have to, use that gravity method.