The "label" text box of the slave template configuration screen (both built-in-template and local-template) is readonly.

Due to this, it is not possible to edit the labels valid for a template. The workaround when creating a new template is to name the template with the desired label as the creation recopies the name of the template in the label field.

https://github.com/jenkinsci/mansion-cloud-plugin/blob/mansion-cloud-1.22/src/main/resources/com/cloudbees/jenkins/plugins/mtslavescloud/templates/SlaveTemplate/configure.jelly#L41