Stop calling the file "file1.xml" for starters, and give it a sensible name.
A good idea is to use the date/ time you downloaded it:
yyyyMMddHHmmss Dogs.xml
This or a similar name means that they are easy for the user to identify and sort by date, and the file names don't clash.
It's also easy to do:
string fileName = @"D:\MyFolder\" + DateTime.Now.ToString("yyyyMMddHHmmss") + " Dogs.xml";
You can then check when you download a file if any of them are out of date:
DateTime tooOld = DateTime.Now.AddDays(-30);
var toRemove = Directory.GetFiles(@"D:\MyFolder", "* Dogs.xml")
.Select(s => new FileInfo(s))
.Where(fi => fi.CreationTime < tooOld)
.Select(fi => fi.FullName);
You can then delete all the files in the collection.