Changing the password in MongoDB for existing user
For v2.4
db.changeUserPassword("app_user", "new password")https://groups.google.com/d/msg/mongodb-user/KkXbDCsCfOs/rk2_h-oSbAwJhttps://jira.mongodb.org/browse/DOCS-1515
MongoDB > 3.X
db.updateUser("root", {pwd: "NewRootAdmin" }) Reference: https://docs.mongodb.com/manual/reference/method/db.updateUser/
You linked to a question asking about adding authentication to MongoDB which involves starting 'mongod' with option --auth. Since you are already running with --auth the restart is not necessary in your scenario.
Just change the user password and you'll be set to go.