Having looked at your program I don't understand your question. Your program seems to receive several different messages and then calls different functions. What do you want to do that is different?
As a separate comment I notice that all your messages begin with "stm1" and the only difference between messages is the final letter. There is no useful information conveyed by the "stm1" part so just leave it out and only send the relevant letter 'A', 'B' , 'C' etc.
Associated with that comment it is not a good idea to use the String (capital S) class on an Arduino as it can cause memory corruption in the small memory on an Arduino. Just use cstrings - char arrays terminated with 0. And have a look the examples in Serial Input Basics - simple reliable ways to receive data.
There is a simple user-input example in Planning and Implementing a Program