Using Java 9+

Path jarPath = Paths.get("/opt/test/myfolder/myinsidefolder/myfile.jar");
Path xmlPath = jarPath.resolveSibling("Test.xml");

Using Java 8 and older

File myfile = new File("/opt/.../myinsidefolder/myfile.jar");
File test = new File(myfile.getParent(), "Test.xml");

Or, if you prefer working with strings only:

String f = "/opt/test/myfolder/myinsidefolder/myfile.jar";
f = new File(new File(f).getParent(), "Test.xml").getAbsolutePath();

System.out.println(f); // /opt/test/myfolder/myinsidefolder/Test.xml

Check out the Java Commons IO FilenameUtils class.

That has numerous methods for reliably disassembling and manipulating filenames across different platforms (it's worth looking at for many other useful utilities too).

File f = new File("/opt/test/myfolder/myinsidefolder/myfile.jar");
File path = f.getParentFile();
File xml = new File(path, "Test.xml");

A more direct approach using only JRE available class File :

String parentPath = new File("/opt/test/myfolder/myinsidefolder/myfile.jar").getParent();
new File(parentPath, "Test.xml");

To rename the file you can use Files.move from java.nio.file.Files

File oldFile=new File("/opt/test/myfolder/myinsidefolder/myfile.jar");
File newFile=new File(oldFile.getParent+"/"+"Test.xml");
try
{
  Files.move(oldFile.toPath(),newFile.toPath());
}
catch (IOException ex)
{
  System.err.println("File was not renamed!");
}