There is no easy way I'm aware of. It's really important when shooting a timelapse to use fixed manual exposure so that you get consistent exposure from shot to shot. It's a non-trivial task to get color to match from one image to the next if exposure doesn't match because you have to worry about not only the black point (darkest point) and white point (brightest point) but also the gamma or rate of change of colors in between.
Additionally the actual light level may have changed while shooting and you don't want to cancel this out. Also, depending on how you had the camera set, it is also possible the white balance may have been changed which would further complicate things.
If it is really, really far off, you could try doing a batch auto-level or auto-curves operation on all of the images in Photoshop and see if that gets a usable look or not, but if the exposure was calculated differently by the camera, then most likely the result from the auto-level and auto-curves will also differ.