String replacement - nice. I rated it homework - cause that's what it is.
String s1="H.R System Finance Marketing";
String s2="Finance";
String s3="Monitoring";
String s4=s1.replace(s2,s3);
You don't need to hold the length of the strings in int variables. Just ask the String on the fly, that is much better as the length is not needed anywhere else.
For the GUI design - yes, stick to Swing for the start. You are using Netbeans - there should be a pretty good WYSIWYG-Editor in there.