How to remove old builds in jenkins?

A better way to remove old builds is discard-old-build plugin where you can specify how many builds you want to keep and all older builds are discarded.

You must either restart Jenkins or reload the data, in order to reconcile the UI with its back end. There is no other way.

If you have access to the "Manage Jenkins" menu there is a link to "Reload Configuration from Disk", which is a little less invasive than a restart.

Keep in mind that the system will be not usable for a few seconds while the data is reloaded, just like during a full restart. If you have a lot of builds, this might take a little. So, make sure that this is ok with the other users of your system.