Spoiler:
First: You cannot reset it, you will have to get yourself a used head unit to fix it.
Second: The anti-theft protection needs to be cleared at a garage who can enter the VIN number in the head unit. Only then will the constant beeping stop. There are tools available for this which I have not tested. I am refering to this video and he seems to be using this tool [1] [2] and software for it.
Third: The new head unit is from a 2018 model C3 and offers functions that this 2017 model does not support. For example, the new unit offers a start-stop function but the original did not. This will then not add this function magically to your car.
Warning:
While this is a super easy job to do, you will do it at your own risk. Think before applying force. Think before doing something permanently.
Parts required:
A similar or compatible head unit (navigation computer)
My old head unit shows the following data on the label:

The new head unit shows the following data on the label:

Tools required:
A fine flat screwdriver to push the pins of the connectors in
A torx 20 screwdriver
Fingernails
Screen removal:
- Pull out the covers on the left and right side of the screen.
- Remove 4x torx 20 screws
- Carefully detach the top two plugs
- For the bottom plug, flip the retainer clip and the plug will come out. To be sure what you are doing, look at the visual guide.
A visual guide can be found here.
Head unit removal:
- Remove the two torx 20 screws that are facing you. One on the left, one on the right.
- Carefully pull the unit out. You have some slack in the wires, so you can guide them out with your fingers. Do not pull too hard!
- Visualise which color plug goes where. The top two ones, blue and orange have a purple line on them. Under those two, you find a blue one. Check in which of the two connectors it is connected to. Next to it is a green one and under that is a black one. And next to the black plug is a white one.
- Start with the blue/purple, then orange, then blue, then green, then black and lastly the white one.
- Remember in which port the blue plug was plugged into.
- Finally, you see a big plug that is held in place by a retainer. Carefully undo the retainer by turning it sideways and take the plug out.
That’s it, now do the thing in reverse with the new head unit.
Next, you need to program the VIN from your car into the radio or it will keep on beeping. It will not end until this is done.
Photos of the unit: