Unharden drywall solution! I take it you mean drywall mud used to tape and texture drywall and its joints. You don't unharden, you tear it out and start over again. I think you need to post this question with more detail so we can better answer what your question
Agree with Kop. Simply scrape it back down below flush, and add another coat. Or punch all of it back into the wall and start over. You will need some support on the backside of the hole. I use wire mesh slightly larger than the hole. I work it in and then hold it in place with a small wire loop then goop up the sides of the hole and wait a day until it sets. Then add other layers to build it up. In a hurry? Use plaster of paris. Sets fast.