You just call it.
exec
public Process exec(String[] cmdarray,
String[] envp,
File dir)
throws IOException
Executes the specified command and arguments in a separate process with the specified environment and working directory.
Given an array of strings cmdarray, representing the tokens of a command line, and an array of strings envp, representing "environment" variable settings, this method creates a new process in which to execute the specified command.
That's what the docs say.
You don't "change the directory" like you when you call "cd" from the shell. You just tell it what directory to start in.
They just mean the program name and the arguments.
So where you might do "cmd.exe /c Start SomeFile", then the tokens are "cmd.exe", "/c", "Start", "SomeFile".
Another version of exec() lets you use one big string (in which case the string is tokenized over whitespace) and also specify the working directory.
Again: read the docs.