Do not use File.Open. Or close it before renaming and then reopen new file.
I've adjusted your function slightly. Your own above which in general looks correct, I just removed unnecessary ifs and concatenations.
string rename;
foreach (var s in yetACCfiles)
{
string source = System.IO.Path.Combine(To_be_processed, s);
string[] spilt = s.Split('_');
string nam = spilt[0].ToString();
int index = yetACCfiles.IndexOf(s) + 1;
rename = String.Format("{0}_{1:00}.csv", nam, index);
string destFile = System.IO.Path.Combine(To_be_processed, rename);
System.IO.File.Move(source, destFile);
}