How to edit using green screen?

Same way you'd use an actual green screen:
-You take a picture with the green screen as the background
-You take a picture of the background you want to replace it with
-You open the green screened picture in some graphics design/photo editing software (You can get GIMP if you don't want to spend money on anything like photoshop, it has most of its features)
-you select the green background in previously mentioned software, make sure the transparent background option is enabled and then you delete the background (It's also best if you manually erase the remaining green bits around the edges to even further increase the quality of the cut)
-After that you paste in your new background (other picture) in a new layer, make sure that it's bellow the layer containing your character/item of focus and then you merge the layers.
-In the end just resize the pic and make any further edits you like (such as adding text or messing around with the colors)

EDIT: You can also do this with videos, both for the background and foreground, but in order to make it look realistic and seamless (So that people can't tell the difference) you will need a good solid color green screen and a strong piece of editing software. I personally use Shotcut for my editing my greenscreened videos (works so well that i wouldn't know the difference between the original background and the new one without prior knowledge)