Steps to Change magento 2 theme:
Steps 1: Log in to Admin Panel.
Steps 2: Navigate to Content > Design > Configuration.
Steps 3: Click Edit theme.
Steps 4: In Applied Theme dropdown, select the theme that you want to change to.
Steps 5: Click Save Configuration
You can view detail here: editing magento themes
Do you want to know more about Magento themes and extensions? Find out more at: