separating the Teddington valve from the manifold is easy. First, secure the gas, then disconnect the lines making sure to not put stress on the valve to manifold connection. Remove the sensing bulb from the oven. Now just loosen the 2 screws under the valve that go into the adapter plate. Carefully pop it loose from the plate and slide it to the rear. It can now be lifted up off the manifold. Be very careful not to kink scrape the thermostatic tube to the bulb on the new one. Move the adapter fittings over to the new valve using a suitable LP/NAT pipe dope. Not teflon tape. Reverse the procedure to replace. Make sure to leak test.
Now you can fire and calibrate .
If you do not own a Manometer and digital recording thermometer, you are best served by hiring a tech that is familiar with gas and ovens. This is not for untrained personnel to accomplish. The sensing bulb mounting, tube connections, and sealing let alone the calibration process takes training.