To restart Jenkins manually, you can follow these steps:

Log in to your Jenkins server.

Click on the "Manage Jenkins" link from the home screen.
Click on the "Reload Configuration from Disk" option to reload the configuration files.
Click on the "Safe Restart" option to initiate a graceful restart of Jenkins.
Wait for Jenkins to shut down and restart.
Alternatively, you can also use the command line to restart Jenkins manually by executing the following command:

sudo systemctl restart jenkins

Note that the exact command may vary depending on your operating system and the way you installed Jenkins. Make sure to check your Jenkins documentation for more information on restarting Jenkins.