So , if you are not able to recognise your user name or password , follow these steps:

1. Stop your jenkins server ,if it is running , using command: sudo systemctl stop jenkins

2. Goto /var/lib/jenkins

3. Open config.xml file with root-edit access.

4. Change the tag from "<useSecurity>true</useSecurity>" to "<useSecurity>false</useSecurity>"

5. Save it.

6. Then run command: Sudo systemctl start jenkins

7. open the jenkins on your browser: http://localhost:8080/

8. Goto: Dashboard > Manage Jenkins > Security

9. Then select:

   "Jenkins own user database" under security realm.

   Select checkbox : Allow users to sign up

   "Loggedin user can do anything" under Authorization.

   And save it.

10. You will be redirected to the login page > select Signup (Create new account)

11. And login with the new account.

12. Goto: Dashboard > Manage Jenkins > Users > Click on the gear icon of the account , whose password you forgot.

13. Change the password and save the changes.

14. Login with the Old account , also you can delete the new created account.

    Note: Dont forget to change the disble "Allow users to sign up" inside Security options.