first U have to get your save onto your PC. When the xport is showing the contents of your mem card, check the box next to the GT3 save, and click Backup. Once that has finished, click the PC data base button, and locate your save. (that can be confusing, and is made easier by 1st deleting all of the GT3 gamesaves in the PC database, and then backing up yours to it.) Once U have located your save in the PC database window, check the box next to it and click "send" and then "file", and select a folder to send it to. Now open MK's,and click on file and read garage, and find and select your save from the folder U sent it to. U should now see your garage in the MKs window, and can edit it. When U have finished editing it, use the write garage function to save it to a new filename. Go back to the xport software, and in the PC database window, delete the save U backed up to there, and then click import and find and select your new file. Check the box next to it and click restore. Select yes when it asks if U want to overwrite your old save, and your done.
Be careful and make sure u are in the PC database window when deleting a save, or U will delete it from your card, and U can only get it back if U have it stored on your PC.