I can only upgrade 1 thing on my gaming laptop. What will enhance performance most?

Since there's no graphics card, and you would have to throw out your MB (and your Windows license) to upgrade the CPU, you are down to RAM or the hard disk. Your move.

Install and run Belarc adviser to get more detailed specifications
http://www.belarc.com/...nload.html
Look for the label on the bottom for an exact model number to further describe what you have.
Ram over 8GB does not help gaming.
The CPU and Graphics processors look soldered down to the motherboard.
http://www.myfixguide.com/manual/lenovo-y50-disassembly/

When the HDD gets filled to much, upgrading to SSD gets expensive and does not help gaming much.

Like the vast majority of laptops, most likely there's nothing to upgrade, except possibly the LCD display to an IPS 1080p if you don't have a touchscreen one.

Laptop gaming performance is far lower than desktop-towers and the towers are upgradable.
Even if possible to upgrade, it is difficult, expensive, and voids all Lenovo support.
This is not different from any other laptop.
Graphics cards, even if a pluggable one, are custom cards and few are upgradable.
The CPUs, when socketed, are spare parts for repair.
Most of the Lenovo Y50's have GTX 860M rank 53 in this gaming table.
http://www.notebookcheck.net/...849.0.html
You might get extra performance adding software as Razer Cortex
http://www.razerzone.com/...com/cortex

I expect there's nothing to upgrade in hardware, but that is no different from other gaming laptops.

As the other long answer says - you can't upgrade the GPU or CPU.

You have enough RAM.

If by "laggy" you mean generally slow frame rates at times, that's a limitation of it being a laptop & having a low-power GPU built in.

If it's glitches - pauses, short freezes - that may be related to hard drive access.
The Y50 can have either a conventional hard drive, a hybrid drive or SSD.

If you do not already have an SSD in it, the first thing to try is run ccleaner to remove junk then do a defrag; that in itself can often get rid some glitches.

A swap to an SSD is about the only thing you can do to that machine, if it's not already got one.
It will improve boot times and general responsiveness, but not improve the basic frame rate capability of the graphics.

The *only* way to get high frame rates with high quality settings on a high definition screen is to go to a desktop machine with a high-end GPU card.