Umbrella API is configured on a per netwowrk basis.
Just go to "Network -> General" within your MX protected network, delete the API credential (on the bottom of the page) and you should be good to go setting it up using the correct key..
I am not a Cisco Meraki employee. My suggestions are based on documentation of Meraki best practices and day-to-day experience.
Please, if this post was useful, leave your kudos and mark it as solved.