Good thread @JOcean linked.  But remember, in the case of the micro it has to be a "T" 35w PSU.

From the Optiplex 9020 Tech Guide: 

This forum post should help. 

@Iron11 

9020 MT/SFF/USFF/Micro Technical Specification
Processors Intel® 4th generation
Core™ i7/i5 Quad Core

(84W for MT & SFF, 65W for USFF
and 35W max for Micro)

9020 technical spec sheet

Intel® Core™ i7-4785T ,   Intel® Core™ i5-4590T

9020 Micro i7 4785T 

thanks all for reply.

I see all the cpu linked by @speedstep are discontinued from intel. 

basically it must be 4th generation max with T and 35W

If you upgraded CPU at this point, it would have to be used or refurb.  Some years ago when I ordered a used CPU thru eBay, it came in brand new condition.