To edit any opportunity listings you've created, first head to the 'Volunteering opportunities' tile within the 'Recruit' section of your admin dashboard.
This will take you to the 'Created Experiences' screen:
Filter by Status or Experience type to find the opportunity you wish to edit if it's not immediately showing, then click the three dot menu to the right of the listing, and select 'Edit' to be taken to the edit screen for that opportunity.
You can also Delete, share, republish or copy (clone) the opportunity from this menu.