Is there any way to fix NES Classic's framerate?

What do you mean?

Is it running slower than normaly? And is the music pitched down and slowed down? If it is, then you might have picked a fake (and dendy region whiich is slower than NTSC) NES classic edition.

And there is no way to fix that

The official NES Classic runs at a flat 60 FPS, while the regular NES runs at about 60.1 FPS, so it is slower to run on and there is no way to "fix" it.