Where is the Transmission Control Solenoid located and how hard is it to replace?

Which one? The transmission shift or control solenoids are not too bad to replace. You will need to remove the transmission pan and filter so you might as well do a transmission service.

Here is a guide to help you:

https://www.2carpros.com/articles/how-to-service-an-automatic-transmission

Here is how to change the pressure control solenoid which is a popular repair. Check out the images (below). Please let us know what happens.