How can I replace hinges

Lenovo gets points in my book for making its service manuals available to the public, and here's a link to the one for your laptop.

Y40 Y50 Hardware Maintenance Manual

This will show you how to remove the hinges, but I have to admit it's a little hard to follow. You can also try searching on YouTube for a video of the procedure; here's one example.

How to replace screen on Lenovo Y50-70 - YouTube

According to the service manual, there are a few different screens being used; the main difference is in whether it has a touch screen or not, but there are also different resolution screens as well. Your best bet is to open up the display and look on the back of the LCD itself. Most laptop manufacturers don't make their own screens, so it's easiest to just find out which one you have and order it by the screen manufacturers part number rather than the laptop manufacturer's model number.

Here's one site that sells screens for your laptop; again, you have to know specifically which screen you need, but prices there range from about $60 to $78 USD. There are many other sites selling them as well such as Amazon, eBay and AliExpress.

Screen for Lenovo Y50-70 20378. Replacement Laptop LCD Screen

Good luck with your repair; feel free to ask if you have any questions and be sure to let us know how it goes!