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Closing and reopening Jenkins issue - please help

I've gotten halfway through this course and had to stop. Trying to return the next day, and while I'm able to launch Jenkins with the "java -jar jenkins.war --httpPort=8080" line, I'm unable to continue. In the browser, it's asking for a log in that I don't recall setting up (although rereading this article shows that I must have done) and no way to reset a password or anything to let me back in to the Jenkins UI.

A google around has shown me to try and find a conflg.xml on my system that just doesn't seem to exist in any of the locations people online have suggested. Is there a way I can clear out everything I've done and start again?

Any advice would be appreciated