{ideas}

First of all you don’t. You have to change the GeneratePassword method used in the ResetPassword method. To do this you need to write your own membership provider and override it like in this example:

public class MembershipProvider : System.Web.Security.SqlMembershipProvider
{
    const String UPPER = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    const String LOWER = "abcdefghijklmnopqrstuvwxyz";
    const String NUMBERS = "1234567890";
    const String SPECIAL = "*$-+?&=!%/";

    public override string GeneratePassword()
    {
        Random rand = new Random();
        String password = "";
        List<String> data = new List<String>();
        for (int i = 0; i < 10; i++)
        {
            if ( i < 3 ) data.Add(UPPER[rand.Next(UPPER.Length)].ToString());
            else if ( i < 6 ) data.Add(LOWER[rand.Next(LOWER.Length)].ToString());
            else if ( i < 8 ) data.Add(NUMBERS[rand.Next(NUMBERS.Length)].ToString());
            else if ( i < 10 ) data.Add(SPECIAL[rand.Next(SPECIAL.Length)].ToString());
        }
        while (data.Count > 0)
        {
            int pos = rand.Next(data.Count);
            password += data[pos];
            data.RemoveAt(pos);
        }
        return password;
    }
}

Note: After you do this you will not be able to use the IIS7 management for your users.