I realised that I put it the wrong date of birth for my Pokemon account and I have already game synced it with my Pokemon um game so I want to change it. So if I remove the game from the global link account will I lose my save file. And If I sync the new account will I lose anything. Please help.
No, you will not lose your save file, its memory is saved on your 3ds, not the global link, so the global link can't do anything to it if you remove the game or not :P