How do I remove an email from the quarantine list?

From the Office 365 Security & Compliance Center:

1. Go to https://security.microsoft.com/.                                
2. Click Review.                              
3. Click Quarantine (any where in the box).
   A list of your emails in quarantine are displayed.
4.  Click any message(s) to select it, then choose from the options given:                                                                  
   • Release                                  
   • Delete message(s)