The way that we show the posts in the home page is base don two filters: Last Post, Recent Activity.
Recent post is showing the newer posts on top and the older ones at the bottom.
Recent activity is bringing the posts to the top, if there is a new activity (edit, like, comment, reply) for that post.
You as an admin can define which view should be the default view for your members. You can set it from the Admin Panel> Settings> Feed> Default Feed Sorting.
Besides that, you can use "Pin" a post to the top of the home page or top of a group page is you want that post to stick to the top. You can unpin it at anytime.