Activities or posts can be edited after they've been posted to the Feed. Recipients of a post cannot be edited. If a post is entirely incorrect or went out to the wrong group of recipients, you'll want to delete the post altogether.
On the Playground mobile app, find the activity or post you'd like to edit and tap the 3-dots to the right of the post. Tap "Edit post" and make any relevant changes. When finished, tap "Save" in the upper right-hand corner.
Navigate to the Feed tab of the Admin Dashboard. Find the post you'd like to edit. Click the three dots to the right of the post and click "Edit".