Use suction cups to pull the glass panel away. It is held by magnets.
Use an external monitor to determine if a display issue is caused by the LCD/cable or a failing graphics card/motherboard. Apple Technician Guide iMac (27-inch, Mid 2010)
Requires pushing through the RAM cavity for easier release. Troubleshooting & Common Issues Use suction cups to pull the glass panel away
4 slots supporting up to 16GB of 1333 MHz DDR3 SDRAM. Apple Technician Guide iMac (27-inch, Mid 2010)
Up to 3.6 GHz Intel Core i3 or i5 (Dual-Core); up to 2.93 GHz Intel Core i7 (Quad-Core).