output text file according the column values

By the way, from the second row.
The first line is a header which is not numeric.

Can you provide a sample, real or replicated, of what the data looks like?

P_HWD	P	BP	NMISS	BETA	SE	
4026	38/492/1483	0.7826	0.03842	125456933	1993	
2589      135/770/1068	0.8619	0.6415	21227772	1952
4016	1/105/1902	1	0.7665	1347325	        1987
1234      424/1024/568	0.3705	0.2921	185118462	1995	

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For example order by column 5.

I want to order the values from small to large

By text length, or actual (converted) value?

By actual (converted) value.

Your sample data appears to be tab-delimited. If so, then the following should be adequate; if not, then we can modify the split operation accordingly.

string[] lines;
string heading;

using (System.IO.StreamReader reader = new System.IO.StreamReader("input.txt"))
{
    heading = reader.ReadLine();

    var sorted = from line in reader.ReadToEnd().Split('\n')
                 let columns = line.Split('\t')
                 orderby Convert.ToDouble(columns[4])
                 select line;

    lines = sorted.ToArray();
}

using (System.IO.StreamWriter writer = new System.IO.StreamWriter("output.txt"))
{
    writer.WriteLine(heading);
    writer.Write(string.Join("\n", lines));
}

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P.S.

As you can see, there is no error checking, so you'll probably want to add some (especially with "Convert.ToDouble()").

If we restrict the value of column 3 within a scope. Can we say

using (System.IO.StreamReader reader = new System.IO.StreamReader("input.txt"))
{
    heading = reader.ReadLine();

    var sorted = from line in reader.ReadToEnd().Split('\n')
                 let columns = line.Split('\t')
                 where(column[3]>1000.00 && column[3]<=5000.00)
                 orderby Convert.ToDouble(columns[4])
                 select line;

    lines = sorted.ToArray();
}

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Thanks.