First:
Prepare a CSV with all issues you want to update, wether they’re already in a resolved « Done » or not. The CSV should contains the following fields:
- Issue key
- Summary
- Resolution
- Resolved
Then, with the following field, go to :
https://<instance>.atlassian.net/secure/admin/ExternalImport1.jspa
You need to have certain privileges to access this page & make updates on issues. Choose « CSV ». That brings you to the following page:

Update your CSV & click Next. You can use the advanced mode to change some import parameters.
In the following step, you can choose wether to update/import issue to a defined project or to have the project defined in the CSV. In my case I have only one project to update.
Then the important part is the date, because you want to update the date 🙂 so be careful to clearly set the Date format mask corresponding exactly to your date format and don’t trust Jira date format by default, it may be wrong (he didn’t parse your file to recognise it ;))
In my case, my date was « 29/09/2019 11:44 » so I had to update the Jira mask to
d/M/y h:m
instead of the
"dd/MMM/yy h:mm a"
proposed by default.

Click on Next. Time has come to map fields

Then click on Next and it’ll bring you to the « Begin import » button.
Click on « Begin import » and wait a bit. Here is the result I got for one story update

Hope it helped cause it wasn’t that easy to find, especially the right place to import the CSV & the date trick due to wrong default value (in my case).
Source:
https://community.atlassian.com/t5/Jira-questions/Is-there-a-way-to-change-the-quot-Resolution-Date-quot-in-Jira/qaq-p/385739
https://docs.oracle.com/javase/1.5.0/docs/api/java/text/SimpleDateFormat.html
https://jira.atlassian.com/browse/JRACLOUD-65502?_ga=2.63511975.921427610.1569990746-1275248540.1512504703