Can you switch out a 256 gb SSD and downgrade to a 1 TB HDD on a lenovo y50?

256 gb is way too small for my taste, I was wondering if I could buy a regular 1 TB HDD from best buy and just stick it in inside of the laptop. Can you do that?

Yes, if it will fit mechanically. But you give no news on the operating system. You will have to transfer the data from drive to drive. Easy Transfer tool or wizard in modern Windows will do that.

What Starrysky does nor seem to realise is that a 1TB drive can have a form factor of 3.5" and 2.5".
A 3.5" will not fit into your laptop
I'm not sure that the easy transfer tool will do the job you want as it will not as I'm aware copy the boot sector 0.
You will need a clone program for that such as EaseUS TODO backup clone section