How to edit GUI FORM IntelliJ IDEA?

Question

Java

Thank you all in advance for your reply!
And sorry if this is a dumb question, but I'm really confused.
The first time I decided to create an Interface (if you can call it that) using the GUI FORM in IntelliJ IDEA, and everything was great until I ran into the fact that I need to change the image of the button during the program.
And through the IntelliJ Form Editor, I calmly set the Image to the button and everything works, but while the program is running, I need to change it, and I can’t do it for 2 days, and today I decided to simply set the Image of the button not through the form, but through the class which is created along with the form, but it did not work for me, it is simply ignored, no errors, nothing, as if this line does not exist.
This is the first question.
And the second is how to see the source code of this file (FORM)?