How to restore a mid-2012 macbook pro to factory settings without losing MacOS High Sierra

Hi -

Can anyone tell me how to wipe and restore a mid-2012 macbook pro to factory settings without losing MacOS High Sierra? I'd like to donate the laptop to an adult assisted living facility. I don't have any install CD's, and I've heard that it's a bit tricky to do this on a macbook pro that has a cd drive without losing the operating system. I also believe I upgraded to High Sierra, it wasn't the OS that the macbook pro came with when it was first purchased. Thanks in advance!