It can be done with some simple commands -
Use this command to keep the original and create a new file with the srt extension. Wildcard helps in renaming all at once.
xcopy *.html *.srt
This command will rename the existing files and will change only the extension.
rename *.html *.srt
For a single file you can use the complete name -
rename hope.html hope.srt
Just like using the rename command, you can also use the move command to rename a file as shown below.
move hope.txt hope.html
Hope it helps. :)