To start with here's the Lenovo web site that has there support files for your system: [http://support.lenovo.com/ie/en/products/laptops-and-netbooks/lenovo-y-series-laptops/y50-70-touch-notebook-lenovo?c=1|Y50-70 Touch].
In reviewing their hardware maintenance manual: [http://download.lenovo.com/consumer/mobiles_pub/lenovo_y40_50_series_hmm.pdf|Lenovo Y40-70/Y40-80/ Y50-70/Y50-80/Y50-70 Touch/ Y50-80 Touch].
Go to page 32 to locate the needed screws to remove the bottom cover. Then jump to 46 on how to remove the memory modules (4 or 8GB DDR3L 1600).
To find out how much memory your system has follow this MS T/N: [http://windows.microsoft.com/en-us/windows/how-much-computer-ram#1TC=windows-7|Find out how much RAM your computer has].