starter replacement instructions please?

Hello, I'm Danny.

It sounds like you might be having an issue with the starter motor solenoid which is a part of the starter motor assembly. Here is a tutorial showing the function of the solenoid and its replacement:

https://www.2carpros.com/articles/how-a-starter-and-solenoid-works

https://www.2carpros.com/articles/how-to-replace-a-starter-motor

Once removed from your vehicle your local auto parts stores will test for free. I've attached picture steps below showing how to remove and replace the starter motor assembly on your truck. Hope this helps and thanks for using 2CarPros.

Replaced starter three weeks later starter solenoid got hot melted plastic part of solenoid. there is no heat shield.

I would check for an air blockage on that side of the engine. unless you are in a very hot area of the country it shouldn't melt the plastic on the solenoid. but you can always find a heat shield to help dissipate the heat. I would look for a quality solenoid as that should be bakelight not plastic.