dropdown onChange update collection

Hello,
Can someone help me with this (I have no experience)

Is there a code for onChange to save drop down selected in a database collection
(drop down is already connected to database collection room choices from which user can choose)

  • Drop down (selected)
  • onChange
  • Save drop down selected in database collection (room april)

2 Database collection

  • room choices to fill drop down with choices
  • Room April to save selected

Thank you very much for helping.
Patricia