If you need to change the url of a page that’s already published, Drupal won’t let you make any changes to the url path settings field of the page itself. You’ll have to change the url alias.
Changing the alias directly will make the page unavailable for around half hour while Akamai changes resolve themselves.
If you can live with that, click on Configuration in the panel at the top of the page
On the configuration screen, click URL aliases
On the List tab of the the url aliases page, find the url you want to change by putting a keyword in the Filter aliases search box and click Filter
Then click edit in the operations column on the right and edit or overwrite the url in the Path alias field
Then click save
Changing the url alias in this way will update the url alias in the url path settings box on the page in question, and untick the Generate automatic URL alias box. So no need to make any changes to the page itself.
Editing the alias will also add a redirect so traffic to the old url will continue to work but only after that deployment window due to Akamai.
Alternatively, if keeping the page live is essential, then you will need to go to config -> url aliases then add a new alias to point at the page (meaning that the one page has two aliases). Then wait for Akamai, once you can see that the new url is live and working you can then go back into the admin and a) remove the old alias and b) go to admin -> config -> url redirects and manually add a redirect from the old url to the new one.