After you remove the three screws (I recommend that you soak them for a while with penetrating lubricant), you may lower the meshed screen into the air plenum and then you will have access to the nuts that retain the vent lid to the hinges. If you want to remove the screen, you will need to remove the lid and then you may pull the screen up and out of the car.
If you remove your lid, pay close attention to any shims that may be between the lid and the top of the hinges as you remove the nuts. Our MK2 had shims on both sides to obtain proper flushness between the top of the lid and the surrounding cowl.
I found that I had to stretch and clamp the replacement seal into place as the one that I received from one of the usual suspects was not an exact fit. I let it sit overnight to let the glue firmly set up. If I did not do this, the meshed screen would have fouled agains the rubber seal and would have rendered the cowl flap inoperable.
Once the seal had dried in place, I then test fit the lid and found that had to re-shim it as the new gasket thickness was different than the original (big surprise). You will find that there is quite a bit of available movement between the lid studs and the hinges so it is very easy to foul the lid on the cowl. I recommend that you apply protective tape around the perimeter of the cowl and the lid to guard against paint chips as you re-set your margins and flushness of the lid to the cowl top. After I had sorted out the flushness shim thicknesses, I removed the lid one last time, placed the screen into the cowl, fastened the lid, adjusted my margins around the periphery of the lid, tightened up the nuts and finally pulled the screen up and screwed it back into place.
The photo below was taken during the re-shimming process that I mentioned above (hence no screen yet).
I hope this helps!