By default, content in collections is sorted reverse chronologically, meaning the content item with the most recent publishing date will be at the top.
However, you can pin any content item to the top of a collection and it will stay there, no matter when it was published.
In order to pin an item, simply follow these three steps:
Hover over it and you'll see an icon with three dots appearing:
Click this item and select "Pin" from the dropdown:
The content item will be pinned to the top of the collection (highlighted by the grey background):