Go to that post/page in the WordPress Admin area.
Right under the title box you should see the Permalink for the post/page.
Click on the "Edit" button just to the right of the link.
Change it from 'welcome-to-my-website' to 'welcome'
(no quote marks are required.)
Click "Ok"
Then click the "Update" button.
You should be good to go!
Cheers.