Can I upgrade from i5 to i7?

It's a lot of work and it may not offer you much gain here. But yes you can upgrade the CPU to a Intel Core i7-2600

For reference here's the i5 versions Apple uses in this series:

• Intel Core i5-2400
• Intel Core i5-2500S

And heres a useful chart outlining the hardware differences from EveryMac: What are the differences between the "Mid-2011" Aluminum iMac models? Which is best for my needs?

So why do I not recommend the upgrade? For most people they won't see any real difference as the clocking is so close between the chips. The only difference is if you are running a deep multi-threaded app like image processing (picture or video) or sound editing. But its big and large projects that will see the biggest differences here.

The other direction here is going with the newer iMac's as they are much more powerful! If I'm doing this for a living I would holdout until I have the money to get either a 5K iMac. Or, if you really can swing it, go for a iMac Pro (10 Core or 14 Core Xeon W), both of these will eat a 2011 iMac as a small snack!

OK I still want to do it! Heres the IFIXIT guide you'll need to follow as we don't have a direct guide: Installing iMac Intel 27" EMC 2429 Dual Drive Kit (HDD or SSD) Follow the steps down to Step31 and continue to remove the logic board fully out. Then jump to this guide: iMac Intel 21.5インチ EMC 2428のCPUの交換 picking it up at Step 43, While a little different the process is the same.

Fully clean the thermal paste and apply fresh paste, then follow the steps in reverse.