title: How to override in phtml file in Magento 2
body: <p>In Magento 2, I have create new theme and copy header.phtml file from vendor/magento/module-theme/view/frontend/templates/html folder and paste in
  app/design/frontend/mage_vender/sample(theme)/module-theme/template/html/header.phtml file.
  And i am trying to change in header.phtml file but changes not showing.
  Please can anyone tell me what step i forgot ?
  I am using magento 2.1.3
  Thank you</p>