Test for existing file, if it exists, rename with date/time prefix. If still exists (rename failed), try to delete.
if (File.Exists(mtxPath))
{
var fiMtxPath = new FileInfo(mtxPath);
var newPath = $@"{fiMtxPath.Directory}\{DateTime.Now:MM_dd_yyyy_hh_mm_ss}_{fiMtxPath.Name}";
File.Move(mtxPath,newPath);
if (File.Exists(mtxPath)) File.Delete(mtxPath);
}