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If the screw is really rusted (I can't imagine it), you can only remove everything around it and hit the screw head with light hammer taps. Spray the bolt several times with a good rust remover spray.
Here's a link (hopefully it works) to a thread I started asking for suggestions. It had some excellent suggestions. Full disclosure, I have not had a chance to try any of the suggestions because I convinced myself the sensor was not the issue. It was the summer so truck started fine and I was afraid of fully committing and breaking the sensor. With this morning's colder weather in NJ, I was once again reminded that my cold start problem did not fix itself. Sooner or later, I will be replacing that sensor and updating the thread, but for now take a look at those suggestions and see if they'll work for you.