请教 "地主家也没余粮" and others 关于信用分数改善

Because of refinancing, I found a serious mistake
in my credit report:

Serious Delinquency

after I checked more details about it, I found it happened 3 years ago with an apartment rent company. When I moved out, the company charged me some money by mistake reporting. I didnot pay it, however, I called the company to argue about that mistake for about 3 months. Later, the company removed it and my balance with it is $0.

I had never thought that company reported it to "Serious Delinquency"!!!

How can I correct this ?

Please help me, because of it my score is bellow 700, and I can't have the right refiance rate.

Thank you very much