Jenkins reset user password :
First stop the Jenkins service if you are already running jenkins
Go to the folder where jenkin’s config.xml file is stored (generally C:\jenkins\.jenkins\config.xml)
Open config.xml file using notepad++ or any text editor
Search for <useSecurity>true</useSecurity> and change that to <useSecurity>false</useSecurity>, Save the file.
re-start Jenkins service (using Java -jar jenkins.war in commandline)
You should not be seeing login prompt but directly accessing jenkins home page when you type localhost:8080 in browser.