Go to C:\Users\Owner\AppData\Roaming\Microsoft\FSX\SimObjects\ and select the folder of the aircraft you want to reset the hours. Open the file STATE.CFG and change the hours to 0.0, then save it.
Some of the folders of the path are invisible, in order to see them, just click on the Organize button in any folder, and then select “Folder and Search Options” from the menu. Click the View tab, and then you should select “Show hidden files and folders” in the list. Just select that, and click OK.