Shell Programming and Scripting

Hello,
I'm an starter in Bash scripting. I would like to write a script in Bash shell that replaces a specific text block (a function) by another text block in a file:

for example in my file --> $HOME/myFile.js
replacing following function between other functions in the file:

Code:

function ABC()
{
  blablabla;
  blablabla;
}

by:

Code:

function ABC()
{
 another blablabla;
 another blablabla;
 another blablabla;
}

without doing any change on other text blocks (functions) in the file. Thanks in advance.