You cannot upgrade just the CPU -- the CPU is soldered to the system board, so the mainboard needs to be replaced.  The cost of the upgrade ($150-200 or more) will be more than the entire system is worth. Also note that if your current system has integrated video, many of the i7  will have the AMD discrete GPU, which will mean purchasing a new power adapter and a new heatsink assembly as well.
Is there anything I should know about upgrading my Inspiron 15 3567 processor from i5 to i7?  
UPGRADING PROCESSOR