Apply Template to an existing issue easily!
Jira Cloud instructions
Click "Apply template" menu item under Issue menu.
Choose "Apply Template" action
Choose Template and click "Apply".
Jira Issue will be updated with data from Template.
Jira Server and Data Center instructions
Choose template from given list
Then Jira Issue will be updated with data from Template
On Server instances - Subtasks will be autopopulated with "Apply" method as well