To do that, you will have to do one of two things:
1) Rename the existing file to something else first - File.Move
] will do that for you, but you may have to check if that file exists as well.
2) Use a different name.
In either case, check if the file exists with File.Exists
To use a different name, I'd suggest that you append a version number just before the file extension - and if you use an integer value to can check if it exists, and increment it if it does, then recheck.
So you would end up with