Note: The Campaigns page require Manager or Account Owner permission level to perform. Learn how to give these permission levels to other users here.
You will see the details page of the campaign page, click on change next to Website text holder:
You will then be warned about the potential risks for updating a campaign's URL, click Update Site if you understand them:
You will then find the Website text holder become editable, enter your new tracking URL, and click Save to finish:
Your campaign's tracking URL is now updated.
Please keep in mind this change will only affect future data, historical data will remain unchanged.