Save the file first...
I'd advise copying the original gamesave file and moving it to a different folder, then saving the edited version as a new file in the Datel/XportV2/savedgames folder.
Open up the Xport software again and click on the "Saves" tab. Up will pop, in the left-hand window, all the various gamesaves on your hard drive, in the Datel/XportV2/savedgames folder.
Click on GT3 GameSave and, in the right-hand window, all the current GT3 Game Saves should appear - I try and keep only one active file in this folder, and all my other game saves are stored elsewhere on the HDD.
Click in the check box in the right-hand window, then click "Restore". This will put the file, slowly, back on your memory card.