overheating, how to replace the internal water pump shaft?

Can you take a look at this diagram? I assume you are looking at number 18.

We don't have a process in the manual on how to remove this shaft specifically but instead on how to replace the pump.

I am attaching this process as well for your review.

Also, here are a couple of guides that will help with doing this repair:

https://www.2carpros.com/articles/coolant-flush-and-refill-all-cars

You will want to pressure test the system afterwards:

https://www.2carpros.com/articles/radiator-pressure-test

Please run through all this info and let us know what else you would need. Sorry we didn't have the exact info you were looking for.