If you go to [http://xonsh.org/tutorial.html#environment-variables] and scroll down to the table, you can see how to change the prompt :)
> PROMPT: The prompt text, may be str or function which returns a str.
If you need non-built-in values in your prompt, just set PROMPT to a function, and call something like `date` within the function. Use `%` to format `date`’s output into a template string, and then pass that template string to `xonsh.environ.format_prompt` to get the final value to return from the function.