How can I enter Mongo as a superuser or reset users?

If you have locked yourself out then you need to do the following:

1. Stop your MongoDB instance
2. Remove the --auth and/or --keyfile options from your MongoDB config to disable authentication
3. Start the instance without authentication
4. Edit the users as needed
5. Restart the instance with authentication enabled

I had a similar issue when I created a user without adding a superuser first. The following steps helped solve the problem:

1. Open the MongoDB configuration file using sudo (sudo vi mongodb.conf).
   The file can be found in /etc/ folder.
2. Comment "auth = true".
3. Stop the MongoDB service (sudo service mongod stop)
4. Start the MongoDB service (sudo service mongod start)

5. Then create "root" superuser using the below command:

   use admin
   
   db.createUser({user:"admin",pwd:"password",roles:[{role:"root",db:"admin"}]});
   

   For reference https://docs.mongodb.com/manual/reference/built-in-roles/#superuser-roles

6. Go back and uncomment "auth=true". Stop and Start/Restart the mongodb service.

If you use a replica set with a keyfile

Security between members of the replica set using Internal Authentication

Keyfile is used for auth in replication.

security:
  keyFile: <path-to-keyfile>
replication:
  replSetName: <replicaSetName>

You can use this command to login as a admin to mongod:

mongo -u __system -p "$(tr -d '\011-\015\040' < path-to-keyfile)" --authenticationDatabase local

Afterword you are able to create or modify your admin user.

Internal Role

__system MongoDB assigns this role to user objects that represent cluster members, such as replica set members and mongos instances. The role entitles its holder to take any action against any object in the database.

Do not assign this role to user objects representing applications or human administrators, other than in exceptional circumstances.