I just did this. My house was prewired though no security system was ever connected to it so just the sensor wires ran to the panel.
First you would need to determine if there are actually contact sensors in the Windows and doors already. The house could be pre-wired but only the wiring and not the sensors. If you have sensors then it’s a matter of connecting the wires in your panel to the screw terminals on the retrofit kit and adding the retrofit kit in the Ring app.
If you don’t have sensors at the end of the wires then you’ll need to add them. This was my scenario. You can check with a multimeter. If you followed along so far then you may be able to DIY.