Individual Properties, Stewardship Sites, Projects, or Contacts are deleted by using Views. However, be aware that Landscape has built-in ways of categorizing these items which enables you to ignore them, but retain the information associated with them. For Properties and Stewardship Sites, you can categorize them as 'Inactive', and for Contacts, you can add an end date to make them inactive. Deleting records is permanent, and usually only used for erroneous entries.
Navigate to Views, and choose the list containing the category you want to edit:
Select the records you want to delete using the check boxes on the left, then click the trash can to delete those items:
You'll be asked to confirm the action twice:
Click 'Yes' both times (as long as you want to permanently delete the items!)
If a Property has an associated Stewardship Site, you'll need to delete that separately. The same with if a Project has an associated Property or Stewardship Site that you want removed.