As above a square Swiss needle file with either a safe (no cut) edge, or stone or grind off the cutting teeth on one edge to make a safe edge.
I recommend buying a higher grade file, not the usual hardware store hobby low quality type.
If you're widening a notch, have the safe edge down so the bottom of the notch is not cut.
If deepening the notch, have the safe edge on one side of the notch and keep away for the other.
Fixed sight revolvers can be targeted by filing the side of the notch to shift the impact over, but be very careful, a little goes a long way and you have to sit down and actually THINK about which side to file to get the group to shift the right way.
Brownell's sell professional quality files that are made for this purpose......
https://www.brownells.com/gunsmith-t...prod25654.aspx
https://www.brownells.com/gunsmith-t...prod25655.aspx
Buy #2.