Replacing symbols with spaces in a string

I'm writing a program that can format text files of thousands of lines of data. The input text file are lines such as:

2010-01-01 00:00:00 0.520368 0.558565 0.567376

What I want to do is replace the dashes and ':' with spaces, and then put proper tabs in. I figured out how to fix the tab problem, but my algorithm for replacement just does not work. Any help is greatly appreciated. My code currently looks like this:

void getReportInfo(ifstream& file, ofstream& ofile)
{
  string date, time;
  double wheight1, wheight2, wheight3;


         while(!file.eof())
         {
           file >> date >> time >> wheight1 >> wheight2 >> wheight3;

           //replace dashes 
           for(int i = 0; i < date.length(); i++) {
             if(date[i]== '-')
               date[i] == ' ';
           }
           ofile << date << " ";

           //replace colons
           for(int i = 0; i < time.length(); i++) {
             if(time[i]== ':')
               time[i] == ' ';
           }
           ofile << time << " ";

           ofile << "\t" << wheight1 << " \t" << wheight2 << " \t" << wheight3 << endl;
         }
}