This way
C#
protected void btnNext_Click(object sender, EventArgs e)
{
Response.Redirect(string.Format("collections/other-fashion?Page={0}", int.Parse(Request.QueryString["Page"])+ 1));
}
VB
Protected Sub btnNext_Click(sender As Object, e As EventArgs)
Response.Redirect(String.Format("collections/other-fashion?Page={0}", Integer.Parse(Request.QueryString("Page")) + 1))
End Sub