If you had the .java file.
You could open this up in Notepad and modify the text. Then use the java compiler to create the class file. If you don't have JAVA installed on your computer you can use the online java compiler here.
http://javaboutique.internet.com/compiler.html
Because you only have the class file.
You can't. Because this is a compiled file. Just like trying to modify an EXE file. You may want to check the documentation that came with the applet to see if you can send a parameter to customize the text. Eg, <PARAM name="text" value="...">.
imaginemn