Jenkins auto created user account, how to login/change password?

Users created by SCM are not "full" users. They are created for purposes of showing SCM changes and receiving e-mails. Therefore they need to sign up (using 'Sign Up' icon that appears to the left of of 'log in' icon in the upper right corner) and provide their password. It is advisable for the username to match the SCM name.

Alternatively, a user with a "full" account can go to http://<jenkins-server>/people/ -> click on username -> click on Configure link to the left, and configure the user (I'm not 100% sure if this will work, though, try it).

With version 1.517, signing up didn't work and gave the "User name is already taken" error.

However you can go to to http://<jenkins-server>/user/<autogenerated username>/configure and set a password for the SCM user, which converts them into a full Jenkins user.