Replace String After SPecifc Chaaracter ?

Here's one way to do it:

using System;

class Test
{
    static void Main()
    {
       string[] sa = 
       {
          "[S]Select Size/Material=10X14 Plastic - $2.62:6.642", 
          "[S]Select Size/Material=10X14 Plastic + $2.62:6.642",
          "[S]Select Size/Material=10X14 Plastic + $2.62:6.642",        
       };
       string replacement = " $3.64:5.523"; // or whatever
       char[] separators = {'+', '-'};
       foreach(string s in sa)
       {
          int index = s.IndexOfAny(separators);
          string t = s.Substring(0, index + 1) + replacement;
          Console.WriteLine(t);
       }

       Console.ReadKey();
    }
}

The output is:

[S]Select Size/Material=10X14 Plastic - $3.64:5.523
[S]Select Size/Material=10X14 Plastic + $3.64:5.523
[S]Select Size/Material=10X14 Plastic + $3.64:5.523

Incidentally, the code assumes there will be only one + or - in the string.

If there could be more than one but you're only interested in the last one, then replace IndexOfAny with LastIndexOfAny.