Swap Local Disk

[DutchTexan]

So I have a 250GB SSD that my machine has the local disk C, and I have a 2TB HDD that my machine has a D drive. Can someone explain to me in a simple way how I make the HDD the main storage device and the SSD just a boot drive? I've watched many videos and I just don't follow. I have changed all the settings I can think of to tell my PC to save to the D drive, but every time I download something or save a file it goes to the C drive. For large applications I dont mind telling it where to go when I download, but I cant track down literally every single thing I have and move it over to the D drive every single time.

 

Is there a simple way to swap the two drives? I can just tell my motherboard to boot off the SSD and make it so that I only tell things to go there. I want the default to be my HDD, but I don't understand how to do that now that it is set up like this already. 

[DutchTexan]

Which part of the first drive is my actual operating system? How would I swap these two? I have tried changing the disk letters and it just doesn't fly. I tried shrinking the largest section of the top drive like some video suggested, but that didn't work either.