No you cannot install i7-3770, an intel 3rd gen cpu in 9020.  Socket is different.  9020 has lga1150 socket which supports intel 4th gen cpu only such as i7-4770/4790.  You should ask seller to upgrade to i7-4770 or i7-4790 for 9020 MT. i7-3770 is c/w lga1155 socket in optiplex 9010/7010. also 9020MT (for mini tower) is different from 9020M(M for Micro). 
1) I purchased a nice 9020 MT with an i-7 processor.from a respectable Dell refurbisher2)  However, it was delivered with just an i5-4670S3) They were very gracious & responsive, & immediately offered to send an i7-3770 to upgrade myself from the i5-4670S.  (I'm quite the novice, but the Dell Youtube video looks simple). 4) Although also offered, for complicated reasons, personally, a full return wasn't an option for me.After reading conflicting information here on Dell, I asked their engineer to confirm that the i7-3770 (in particular) was a compatible upgrade, & they immediately responded: "The system will run with both the i5 and i7 without issue."  So just being cautious (and going right to the source), should the i7-3770 be OK?Again as a novice, I'd like to confirm the switch before "cracking the case." Respectfully,//Gary
Dell Optiplex 9020 MT CPU upgrade - i5-4670S to i7-3770