Hello there :) You can do this via the website builder. Here you click on pages > manage pages and change it. It also let you set up an automated redirect. Alternatively you can go to the website app and there to Configuration > Pages. There you click on the page in question and change the URL in the field "Page URL". 

But be aware, that if you change "mysite/contactus-thank-you" to "mysite/gracias", the actual contact form will still redirect to the old page ("contactus-thank-you").  That is of cause, if your form is set to "redirect" in the field "On success".

In that case, to change that, you go to your contact page, click edit and click on the form. On the right you look for the field "URL" where you have to change it to the new one, "/gracias".

I hope that helps!

From website app -> go to Configurations -> Pages 

Open the page and change the Page URL as below:

for  "mysite/contactus-thank-you" page  if you changed the url to " mysite/gracias" then you need to change it in the contact us view as well as below: