The new 1040, replacing my 1030+ shows "0" milage in the odometer fields in all three activity profiles. Like most, I want the actually milage on the bikes in the odometer fields to facilitate maintenance, tire change, etc., not "0". After many hours on forums and several calls to Garmin support I finally figured out how to do this and thought to share with others to save hours of time and frustration.
I was able to change the odometer when I went from the 1030 to the 1030+. It requires editing the "'Total.fit" file which I figured out how to do in a few hours. The 1040 is different.
First, my Apple desktop will not show the 1040 as an external drive like the 1030 did. Garmin suggested an app for the Apple to read the 1040 as a drive. I tried this and found the 1040, but could not open and edit the Total.fit file. So, best to use a Windows system. I used the Windows 10/Parallels partition on my Apple. It still took hours and several app suggestions that did not work. Below is what finally worked for me with Windows 10. There are many steps and not straightforward, but you can benefit from my trial and error.
1. In Windows, find and download a free app called "Fitedit" and open it.
2. Connect the 1040 to the USB port.
3. With Fitedit, locate the 1040 and import the "Total" folder, and go to "Data" in the app.
4. You will see about 11 lines of data, mostly zeros if you have not yet used the 1040 and several activities called cycling. It does not show you the activities names. This complicates things since you do not know which activity profile distance you want to edit. In my case, I rode 20 miles under a certain activity. Under Distance one line showed showed a number around 32,000, the other lines were zeros. On my phone unit conversion app I entered 20 miles and down the list under meters and lo and behold, it read around 32,000. So the distance in the "Total" file is in METERS and I identified the Activity I wanted to edit.
5. Now, convert the miles you want to enter into meters, it will be a very big number if the miles are in the thousands, and enter this number in the distance field, then Save the file (it will be named something like, "Total-1. Now Export the file. You will be tempted to export it directly to the 1040 drive. It MUST be exported to the "Newfiles" folder. But the direct export did not work for me. Instead I exported the new "Total-1" file to the desktop. Then copy/moved the Total-1 file to the Newfile folder in the 1040 drive.
6. Eject the 1040. It will restart and when you open the Activity Profile and look at the odometer field you will see the correct milage from your old bike and you will be so happy. I shouted!
For other Activity Profiles just do a short ride, a few tenths of a mile, and convert the distance in meters so you will know which line to edit, and repeat the steps above.
....or, Garmin can do a software update that has an "edit odometer" function.