Done 
In case anyone is wondering about this, it’s one of the nicer setup things you can do for the instrument, and it’s super easy.
First, check it to see if it is high. The way I do this is fret in the third fret up against the second fret wire (i.e. in the notes G/C/F/A#, but on the other side of the fret).
The string should barely clear the first fret, just a tiny amount. I usually just tap it with my index finger over the first fret wire, it should move but just barely move. If there’s significant play, the nut is too high. Check all the strings; if they are all too high, as is usual, then you can sand; otherwise you need to file. I’m going to assume they are all high here because that’s the usual case (and was on my BB tonight).
So, first, loosen the strings and pop off the nut.
For new instruments this should pop out easy as most manufacturers barely tack them in. Used ones you might get a case where a prior owner glued them in with a lot of glue. That sucks. There’s youtube videos to help you there, I like StewMac’s.
The next step is to lower the nut by sanding material off the bottom. This nut is flat on the bottom. That’s good, that makes this much easier. For flat nuts like this, simply put a piece of 150-220grit sandpaper flat on a table and go at it a while.
Just apply even pressure and sand the bottom of the nut until a little material has come off. Then clean it off, pop it back on the bass, tighten the strings, and check the nut height for all atrings again. Repeat this until it’s the right height for the lowest string:
Usually they are all roughly going to end up in the same ballpark so you can stop here. If any are still too high, you can file, but that’s another TED talk.
Much much nicer. Proper nut height makes a really big difference to the feel of the bass. You don’t need to glue it back in, the strings will hold it down nicely. I do like to tack it in place myself with a tiny amount of super glue, but that’s mostly just for convenience so you don’t lose the thing while changing strings. Do not use more than a tiny drop of glue, no more than like a 1mm drop is needed.
If your bass happens to have a radiused (curved) nut, it’s a little trickier. What you do then is use the fretboard as a sanding block and wrap the sandpaper around the fretboard. I recommend checking out a good video on this.