Reset Jenkins Admin Password

Make a backup copy of a Jenkins config file (If need to restore the previous settings)

cp -r /var/lib/jenkins/config.xml /var/lib/jenkins/config.xml.back

Disable the security

vim /var/lib/jenkins/config.xml 

<...>

<useSecurity>false</useSecurity>

<...>

Restart the Jenkins service

systemctl restart jenkins

Go to the Jenkins UI (No credentials this time) and reset the admin password

Navigate to "Manage Jenkins" 

In Security "Configure Global Security"

In Security Realm Select "Jenkins’ own user database" and  Save

Go to "People" From the Dashboard 

Select  username and "Configure"  Enter a new password in the "Password" and "Confirm password" fields and Save

Once the admin password is reset, restore the /var/lib/jenkins/config.xml file and restart Jenkins

 mv /var/lib/jenkins/config.xml.back /var/lib/jenkins/config.xml

 systemctl restart jenkins