Step-by-step guide

  1. goto 

    [jenkins-instance]/script

  2. insert this script and update the job name

    item = Jenkins.instance.getItemByFullName("your-job-name-here")
//THIS WILL REMOVE ALL BUILD HISTORY
item.builds.each() { build ->
  build.delete()
}
item.updateNextBuildNumber(1)

CANNOT BE UNDONE

To delete all build of all jobs use:

for (item in Jenkins.instance.items) {
  item.builds.each() { build ->
    build.delete()
  }
  item.updateNextBuildNumber(1)
}