I was stuck but found the answer on the Strapi forum.
At root of your project, run this command:
npm run strapi admin:reset-user-password --email=existing@user.com --password=NewPassword
(You'll need to use real values for your email and password flags)
Once you've ran that command, you’ll be prompted to input the same details again eg:
[? User email? existing@user.com
[? New password? [hidden]
[? Do you really want to reset this user's password? Yes
(These need to match the email and password you used on the previous command)
Finally you'll need to run
strapi build
Once it's rebuilt, you'll be able to start Strapi and log into the admin with you new details!
Original instructions/tread: https://forum.strapi.io/t/reset-strapi-admin/6136