If you know what you deleted, you leave your current database open, and double-click the backup. It will open in a separate window with a blue stripe at the top that says “OmniFocus Document.” This differentiates it from your master database.
Navigate through the backup and find the things you deleted. Copy and paste them back into your master database, then close the backup.
If you don’t know what you deleted, you may be out of luck, unless you can figure it out using search or sorting.