how to increase my RAM from 2GB to 8GB in windows 7 64-Bit operating system?

Just buy a matched pair of sticks. Windows will automatically recognize the memory. Nothing else to do. Just make sure the memory is on the compatibility list for your board before you buy it.
 
Just go to downloadmoreram.com! /s

Check if you have any open slots. You may be able to buy a 6GB kit and mix memory, but it may also be a better idea to just completely replace what you have. What computer/motherboard do you have?