I got Poke'mon Ranger as a second hand game at Games Stop. And I wanna get the Manaphy egg to send to my Pokemon Platinum. But, the previous owner already sent it to his/her game! Cuz everytime I open the Manaphy Mission, the Assitant talks about how she is worried for the Manaphy egg they sent to Sinnoh. So, is there way to reset the data to play the Mananaphy Mission again?

Unfortunately, no. You can't restart it (otherwise people would have multiple manaphies) and what's even meaner is you can't even replay the mission. You'll have to get a manaphy else where.