"iMovie can’t be installed on “Macintosh HD” because Mac OS X version 10.9.2 or later is required."

Hi,

I recently bought imac system for my work. System Configurations are as below

iMac

27-inch, Mid 2011

Processor 3.4 GHz Intel Core i7

Memory 8 GB 1333 MHz DDR3

Graphics AMD Radeon HD 6970M 2048 MB

Software Mac OS X Lion 10.7.5 (11G63)

I was trying to install imovie from apple store but I am getting following message

"iMovie can’t be installed on “Macintosh HD” because Mac OS X version 10.9.2 or later is required."

Please help? What should I do? I am new to mac.

I will appreciate your response.