If you want to dominate in the circle, I suggest you start mastering your change-up. To do so, it's important to practice your change-up regularly. In my previous spin series article, I mentioned the importance of selling your change-up and creating deception. To do so effectively, you'll want to throw this pitch for a called strike, and generate swings and misses. Below are some of my favorite drills to develop a devastating change-up. I almost hesitate to call them drills, as these exercises are more about changing visuals and using constraints to improve the trajectory of the pitch.
By now, you've probably realized that I love using strings to work on pitch depth and movement. For the change-up, there are two exercises that I love to use the strings for. Both will involve a mid-distance string, and the exercises can be used interchangeably, as they're relatively the same. I have found that some pitchers respond better to different goals, so try out the thought process for both and see which works best for you.
I originally used this video for other movement pitches like drop and rise, but you should try to tunnel each pitch you throw. If every pitch looks similar as it's on the way to the plate, the hitter will find it difficult to hold back their swing. This drill is ideal for chase pitches or creating swings and misses with your change-up.
Change-ups are unique to the individual, and what works for someone else might not work for you. Take the time to find the change-up that's right for you, and practice it CONSTANTLY. In the same way, you have to find out which drills and exercises work for your specific change-up, so take the time to try out a few. Once you find something that works for you, stick with it and make it part of your daily bullpen routine.