After receiving the message, can the sender modify the content of the message I received?

No, the information has been transmitted to your receiving device's mobile phone by signal, and he can't change the information on your mobile phone. Except for the hacker's remote control, he will delete your message, but he will not change it. This is extremely difficult, and it is impossible for hackers to do such boring things without interest or dissatisfaction. So, to sum up, don't worry, it can't be changed.