Loop, Timer & datagridView

Question

I have a Timer that managed the loop. In the loop  I have datagridview.Rows.Add( b, a);
The code of the loop is as follow:
int n = 5
for ( int m=0; m<=n ; m++)
{
  datagridview1.Rows.Add( b, a)    // Just an example

}

With the timer the datagridview will be filled up.
Mine Question is: What Can I do that after the 5 loop it start back on the first
Row of the DatagridView?

It look like overwrite the first 5 Rows  that came up. I have too look only 5 rows in the run time