I want to test malware that wrote in .NET 4.5 and obfuscated by Confuser 1.9.
I have tried to open it with .NET Reflector, ILSpy and dotpeek, but all of them can't open it.
How can I debug (and modify) it? There is a special tool for that?
Try http://de4dot.com its a powerful .net deobfuscator. I've authored a serie of tutorials dubbed "demystifying dot net reverse engineering" google it, its a great point of start if you are new on .net RE.
Here is an article on how to deal with obfuscated assemblies http://resources.infosecinstitute.com/reverse-engineering-obfuscated-assemblies/
And this is directly related to what you asked for : [.NET] Decrypt Confuser 1.9 methods : http://fr.scribd.com/doc/207710371/NET-Decrypt-Confuser-1-9-Methods#scribd