HEIC converter to jpg that will retain creation/modification date

My fiancé passed away several months ago. His aunt was the Executor of his will so I had to send his iPhone to her so she would have access to his financial information. Before I sent it to her I copied all his pictures onto my computer. I'm not an iPhone user. I was trying to do some editing of some of the photos yesterday and realized they had some other extension .HEIC, not jpg. I had to do some research to even figure out what that was. The creation date is showing the date I uploaded them to my computer. The modification date is the date the photo was taken. All I really care about is maintaining the original date of the photos, the date modified. I have done research and tried several different options but none seem to retain the data I want. I did find one program that I downloaded, i Mazing Convertor, which is retaining the creation date but that is not the date modified, the date of the photo. And that date is the day I uploaded the photos to my computer. What can I do to retain the modification date (the date of the photo) and convert them to jpg? Reminder I do not have the phone, I'm not an iPhone user nor am I real technologically savvy. Maintaining the original date (modification date) is very important to me as this is the only copy of these photos I will ever have.