Rename based on file name

Hello
I'd like to do a task. I have ( or not have, depends on user select ) a number file list like:

 
File_1.txt
File_2.txt
File_3.txt
File_4.txt
File_5.txt

If a user delete a file, like:

 
File_1.txt
File_2.txt
File_4.txt
File_5.txt

I want reneame the other file list, in this way

 
File_1.txt
File_2.txt
File_3.txt  (Previusly was File_4.txt)
File_4.txt  (Previusly was File_5.txt)

I don't know how to check if file exist or not based on the number or file, then rename the rest of the list. Thanks