If you want to change the title of a post type you must do the following:

Go to Content >> Edit Post Settings

Find the post type that you want to rename and click on Edit, for this we are going to use the Website Blog Article Post Type as an example.

General Settings Tab (Your unique name for this type of post):

Update the URL to match the name:

Profile Page Design Tab (This is the title that is going to show up on the Member's Profile):

Search Results Design (This title is going to appear when accessing the actual search results visible to the visitors and other members)

Lastly, click on Save Changes.

When Saving the Post Type you are going to see a message for you to confirm this change: "The URL of this Post Type is being changed from blog to diary. This action will also update the slug URL of all associated posts." 

This means that if you already had some posts and those were working with /blog now these are going to display with /diary.

Make sure to always refresh the cache of your website to see the changes: 

Refreshing The Cache Of Your Website

Now if you visit this page you can see the new Title and URL: