If you want to reshape the top of the nut to a 12" radius then it's easy enough to do, but you will then need to re-cut/deepen some of the string slots. This is assuming the nut is tall enough to allow re-radiusing. Just use a 12" radius sanding block with care to ensure a balanced taper on each end of the nut, or failing that (if you're game)...

Download some radius gauge templates, print out a 12" radius one (make sure it's 100% scale) and use that to mark out and cut the new radius on your nut (see below).

Or...

Get yourself some masking tape and put a couple of strips on a table. Tie a piece of string to the tip of a pencil, measure 12" from the tip and mark it on the string. Put the pencil tip on your masking tape, hold the string at the 12" mark and scribe an arc on the masking tape... you now have a 12" radius guide. Trim the tape on the outside of the arcs then stick it on the fretboard side of your nut with the arc level with the peak of the nut (in the centre) and an equal taper to each end. Repeat on the other side of the nut so you have a guide on each side. File the nut down close to the arc line, then clean up with sandpaper and do final shaping. You'll then need to tweak your string slot depths etc.

An alternative is to use the pencil/string method to drawn an arc on some thin cardboard or plastic, cut that out and then use it as a template to draw/scribe 12" lines onto the nut, but the tape option allows repositioning and may end up being easier to manage (and easier to see on a black TUSQ nut).