From the left sidebar navigate to 'Portfolio' and click 'Pages'

Hover the right side of the current Page Title until the '…' icon appears, click to open the dropdown menu

Click 'Rename'

Current Page Name

Edit the Page Name (in this screenshot it is for a Gallery Page)

Edit the Page URL, the unique address of this page, an error will be shown if it matches an existing page, this should always be typed in lowercase with a hyphen between words)

Click 'Update'

Click on the Page Name to edit

Edit the Page Name (in this screenshot it is for a Custom Page)

Edit the Page URL, the unique address of this page, an error will be shown if it matches an existing page

You can also access this, and more under the 'Settings' link (see screenshot below)

Click 'Update'

Access the 'Settings' pane

Edit the 'Title'

Edit the Page 'URL'

You will be prompted to 'Save Changes' before leaving this area