Please I need to delete and repost my old posts on WordPress so that they can be reindexed by search engines like new content. I don't want to manually do it. Please can someone help with an idea of how to?
To do this you update your posts' post_date values. There are plugins for the purpose.
But be careful. Search engines like Google are really good at detecting attempts to play games with them to boost search rankings. People have been playing those games since Yahoo ruled the search-engine world. If they detect such things on a site they down-rank it or even stop showing it altogether. Simply updating dates without changing content may trigger a game-playing filter. You don't want that.
Ask yourself the question, "how do I make my site more useful to people who search for it?" New and updated content is a good way to do that.