Pokémon Diamond/Pearl Forum

I am in Japan and just bought a DSi and Pokemon Diamond (used, which is an important factor which I will flesh out here in a second). I speak a little Japanese but dont understand most of the language. I have been able to get by pretty well except I can't save and I don't know why! 
 
I understand where the save option is in the menu, for those of you who speak japanese better than I do this is what it says when I try to save: すでに　べつの　ポケモンレポートが　かかれていますので　レポトガ　かけません。　くわしくは　とりあつかいせつめいし ょを 　ごらんください。 
 
But I think it means there is already a saved game so because of this I cannot save. Then it definately says to please refer to the instruction manual, which i will try to do but I have failed at thus far. I bought this game used so it already came with a saved game on it. Anyone know how to save over/delete already existing saves on pokemon diamond?

K. Do you play diamond? How would I go about freeing a saved slot up (the one and only?)