While your chest build ups from two big muscles the pectoralis minor and mayor, you can target the parts with various exercises.
The most important exercise is without a doubt the bench press that helps to develop both in size and strength. Hence, it must be in your workout routine.
But, we come with different genetics, and almost everyone has a weaker point. For example, my upper chest develops well while my lower chest needs more training.
Ways to target the different areas of your pecs
Firstly, by changing the grip. Wider grip presses work the outer parts while close grip exercises work the inner section better.
Another technique is by altering the angle. Flat presses works the middle part, incline ones the upper chest and decline exercises are good for the lower part.
Also, it is a good idea to change the equipment you use. I prefer free weight training, so I mainly use barbell and dumbbells, but machines such as a cable crossover can also help to shape your chest.
Tip: The same is true if you prefer bodyweight exercises instead of lifting weights.
I know there are a lot of chest workout routines online, but I believe we have to test out what works us. Hence, you need to try various exercises and find the ones that get you the best results. It requires time, but at the end, you will have a program which helps you to get rid of all your weakest points effectively.
For beginners: If you have just started muscle building, I say do not care about the parts. Firstly, you need to gain muscle mass and strength to have something to work on. So, stick to the flat bench presses and other chest exercises with free weights to grow.
OK, now check out the following infographic from which you can learn different exercises to target one particular part of your chest.
To sum up
You can get rid of the weak point of your chest by doing the right type of exercise(s). Do more sets and reps, and the results will come sooner or later. And at the end you will have perfectly-shaped pecs you will be proud of.
Note: Most of the people in the gym like focusing on the muscles or exercises that they are already good at. It is evident since they always have a sense of achievement. But, that is a big mistake. You must focus on your weak points to get rid of them. In the beginning, it is not fun, but that is how you can get a symmetric and balanced body.