How do I change the filename number without changing its whole name?

E.g.:

13 - Jesu, joy of man's desiring
14 - High and lifted up

I'd like the names to be

1 - Jesu, joy of man's desiring
2 - High and lifted up

I have about 200 files and I can't go one by one. Please, help me.

Try the function Convert>Tag-Tag for _FILENAME (you have to type in the field name)
Format string: $num(%_counter%,1) - $regexp(%_filename%,\d+ -,)